Chapter V: Muscles of the Leg and Foot (22)

2. The =ectosylvian fissure= has approximately the shape of an inverted u, and curves over the lateral fissure. It is regarded as consisting of three parts—anterior, middle, and posterior (F. ectosylvia anterior, media, posterior).

3. The =suprasylvian fissure= lies above and approximately concentric with the preceding. It also consists of three parts—anterior, middle, and posterior.

4. The =ectomarginal= or =collateral fissure= runs forward on the dorsal aspect of the hemisphere almost parallel with the dorso-medial border. A little in front of the middle of the surface it gives off a short =ansate fissure= (F. ansata) which runs obliquely inward and forward, and is continued by the coronal fissure.

FIG. 542.—CEREBRUM OF DOG, DORSAL VIEW.

_a_, End of lateral fissure (of Sylvius); _b_, ectosylvian fissure;
_c_, _c′_, _c″_, suprasylvian fissure; _d_, fissura ansata minor;
_e_, ectolateral fissure; _f_, ectomarginal or collateral fissure;
_g_, fissura ansata; _h_, coronal fissure; _i_, medilateral fissure;
_k_, entomarginal fissure; _l_, postcruciate fissure; _m_, cruciate
fissure; _o_, presylvian fissure; _p_, fissura prorea. (After
Ellenberger, in Leisering’s Atlas.)
]

FIG. 543.—FISSURES OF LATERAL SURFACE OF CEREBRUM OF DOG, RIGHT SIDE.

_a_, Fissura prorea; _b_, frontal fissure; _c_, olfactory fissure;
_d_, _d′_, sulcus rhinalis; _e_, presylvian fissure; _f_,
precruciate fissure; _g_, postcruciate fissure; _h_, cruciate
fissure; _i_, lateral fissure (of Sylvius); _k_, _k′_, _k″_,
suprasylvian fissure; _l_, _l′_, _l″_, ectosylvian fissure; _m_,
fissura ansata minor; _n_, ectomarginal or collateral fissure; _o_,
fissura ansata; _p_, coronal fissure; _q_, ectolateral fissure; _r_,
medilateral fissure; _s_, entomarginal fissure. (After Ellenberger,
in Leisering’s Atlas.)
]

FIG. 544.—RIGHT CEREBRAL HEMISPHERE OF DOG, INNER SIDE.

_1_, Corpus callosum; _1′_, genu, _1″_, splenium of corpus callosum;
_a_, genual fissure; _b_, splenial fissure; _c_, suprasplenial
fissure; _d_, hippocampal fissure; _e_, sulcus corporis callosi;
_f_, postsplenial fissure; _g_, lesser cruciate fissure; _h_,
cruciate fissure. (After Ellenberger, in Leisering’s Atlas.)
]

5. The =coronal fissure= is the continuation of the ectomarginal. It runs forward and downward, curving around the cruciate fissure, and ends behind the middle of the presylvian fissure.

6. The =ectolateral fissure= lies between the posterior part of the ectomarginal fissure and the suprasylvian fissure.

7. The =entomarginal fissure= lies between the anterior part of the ectomarginal fissure and the dorso-medial border. It is often indistinct and not rarely absent.

8. The =presylvian fissure= begins at the sulcus rhinalis, a little in front of its middle, and curves over the anterior third of the lateral surface of the hemisphere in front of the coronal fissure.

9. The =cruciate fissure= is the deepest and most characteristic. It cuts deeply into the anterior third of the dorso-medial border and runs almost straight outward. It is continuous on the inner surface of the hemisphere with the calloso-marginal or splenial fissure.

10. The =sulcus rhinalis= is continued on the tentorial surface of the hemisphere by the occipito-temporal fissure. Anteriorly it is prolonged as the olfactory fissure, which is concealed by the olfactory bulb and tract.

FIG. 545.—DEEP DISSECTION OF HEAD OF DOG, SHOWING ESPECIALLY
TRIGEMINAL AND HYPOGLOSSAL NERVES.

_a_, Hypoglossal nerve; _b_, cervical branch of _a_; _c_, mandibular
division of trigeminus; _d_, lingual nerve; _e_, nerve to
submaxillary gland; _f_, deep temporal nerve; _g_, pterygoid nerve;
_h_, buccinator nerve (cut); _i_, inferior alveolar or dental nerve;
_k_, staphyline branch of lingual nerve; _l_, chorda tympani; _m_,
mylo-hyoid nerve; _n_, sphenopalatine nerve; _o_, lesser palatine
nerve; _p_, great palatine nerve; _q_, infraorbital nerves; _r_,
subcutaneus malæ; _s_, branch of oculomotor nerve to inferior
oblique muscle; _t_, lacrimal nerve; _u_, frontal nerve; _v_,
trochlear nerve; _w_, abducens; _1_, carotid artery; _2_, lingual
artery; _3_, internal maxillary artery; _4_, thyro-pharyngeus
muscle; _5_, hyo-pharyngeus muscle; _6_, thyro-hyoideus; _7_,
sterno-hyoideus; _8_, baseo-glossus. _9_, genio-hyoideus; _10_,
genio-glossus; _11_, stylo-glossus; _12_, pterygoideus internus;
_13_, outline of submaxillary gland (dotted); _14_, atlas; _15_,
bulba ossea; _16_, zygomatic arch (dotted); _17_, rectus oculi
inferior; _18_, obliquus oculi inferior. (Ellenberger-Baum, Anat. d.
Hundes.)
]

11. The =calloso-marginal= or =splenial fissure= is parallel with the splenium and middle part of the corpus callosum on the inner surface of the hemisphere. It is continuous in front with the cruciate fissure and runs downward and outward across the tentorial surface to join the occipito-temporal fissure and sulcus rhinalis. It gives off from its highest part the short lesser or accessory cruciate fissure.

12. The =suprasplenial fissure= runs almost parallel with the posterior part of the calloso-marginal fissure on the inner and tentorial surfaces of the hemisphere.

The =olfactory bulb= is large and is strongly compressed laterally; its anterior end projects beyond the frontal pole of the hemisphere. The tract is short and is also flattened laterally. Both contain a narrow cavity which communicates with the lateral ventricle. The =pyriform lobe= is large and rounded.

The =cranial nerves= present the following special features which are worthy of notice:

The =ophthalmic nerve= gives off lacrimal, frontal, long ciliary, ethmoidal, and infratrochlear branches. The =lacrimal nerve= arises from the ophthalmic at its origin;[215] its recurrent branch emerges at the orbital ligament and concurs with the zygomatic and frontal nerves in the formation of the anterior auricular plexus. The =frontal nerve= emerges from the orbit in front of the upper end of the orbital ligament, ramifies in the upper lid and the adjacent skin of the forehead, and sends branches backward to the anterior auricular plexus. The =long ciliary nerve= accompanies the optic nerve and divides into several branches which pierce the posterior part of the sclera. The =ethmoidal nerve= gives off internal nasal branches and ends in the muzzle. The =infratrochlear nerve= runs forward between the internal straight and the superior oblique muscles of the eye and ramifies on the face in the vicinity of the inner canthus.

FIG. 546.—NERVES OF FACE OF DOG. PAROTID GLAND IS REMOVED.

_a_, Facial nerve; _b_, posterior auricular nerve; _c_, internal
auricular nerve; _d_, digastric nerve; _e_, inferior buccal nerve;
_f_, cervical branch of facial; _g_, auriculo-palpebral; _h_,
superior buccal nerve; _i_, temporal branch; _k_, _k′_, _k″_,
zygomatic branch; _l_, auriculo-temporal; _m_, malar branch of _l_;
_n_, buccinator nerve; _o_, mylo-hyoid branch; _p_, subcutaneus
malæ; _q_, lacrimal nerve; _r_, frontal nerve; _s_, infratrochlear
nerve; _t_, infraorbital nerve; _1_, paramastoid process; _2_,
digastricus muscle; _3_, base of concha; _4_, masseter; _5_,
zygomaticus; _6_, scutularis; _7_, zygomatic arch; _8_, maxilla.
(Ellenberger-Baum, Anat. d. Hundes.)
]

The =maxillary nerve=, after giving off the zygomatic, divides into infraorbital and sphenopalatine branches. The =zygomatic nerve= emerges through an opening in the upper part of the orbital ligament and ramifies in the lower lid and on the adjacent surface of the face. The =infraorbital nerves=, two in number, divide within the infraorbital canal and after their emergence upon the face, thus forming seven or eight =external nasal= and =superior labial branches=. The =sphenopalatine nerve= gives off =lesser= and =greater palatine= and =posterior nasal nerves=.

The =mandibular nerve= passes out through the foramen ovale. The =superficial temporal nerve= divides into auricular, temporal, and malar branches; the last-named crosses the masseter and ramifies with the buccal nerves. The =mylo-hyoid nerve= is given off from the mandibular nerve almost immediately after the latter emerges from the cranium; it innervates the mylo-hyoideus and digastricus and gives off a branch which turns around the lower jaw, joins the inferior buccal nerve, and ramifies on the lateral surface of the face. The =inferior alveolar nerve= arises by a common trunk with the mylo-hyoid; it gives off dental branches and terminates in mental and inferior labial branches. The =lingual nerve= supplies vasodilator and excito-secretory filaments to the submaxillary and sublingual salivary glands; these fibers are derived from the chorda tympani.

The =facial nerve= divides near the posterior border of the jaw into four branches. The upper branch is the =auriculo-palpebral nerve=, which divides after a very short course into anterior auricular and zygomatic branches. The latter curves upward and forward across the zygomatic arch toward the eye, and divides into branches which supply the eyelids and nasal region and concur with the frontal and lacrimal nerves in forming the anterior auricular plexus. The =superior buccal nerve= accompanies the parotid duct across the masseter. The =inferior buccal nerve= runs forward along the lower border of the masseter and the mandible. The two nerves ramify on the cheek and anastomose with each other and the infraorbital nerves to form a plexus from which branches go to the muscles of the lips and nostrils. The =cervical branch= runs downward and backward over the submaxillary gland and ramifies in the cervical panniculus; it communicates with the inferior buccal nerve and sends twigs to the parotido-auricularis muscle and the submaxillary space.

The =vagus= bears a =jugular ganglion= and a =ganglion nodosum=. The latter is situated near the superior cervical ganglion on the rectus capitis anterior major and dorsal to the carotid artery; it is fusiform and may be about half an inch (ca. 1 to 1.5 cm.) long in a large dog. In its course in the neck the nerve is inclosed with the sympathetic trunk in a common sheath and is related ventrally to the common carotid artery. The formation of dorsal and ventral =œsophageal trunks= by the union of the dorsal and ventral divisions of the two nerves takes place at the hiatus œsophageus. The =dorsal trunk= concurs with branches of the solar plexus in forming the =posterior gastric plexus= on the visceral surface of the stomach. The =ventral trunk= ramifies on the parietal surface of the stomach, forming the =anterior gastric plexus=, from which a considerable branch passes along the lesser curvature to the pylorus. Some special features of the collateral branches are: Two pharyngeal branches are present. The =superior pharyngeal branch= arises from the vagus above the ganglion nodosum. It passes under the deep face of the carotid artery and descends on the lateral surface of the pharynx to end in the crico-thyroid muscle. It furnishes a root of the inferior pharyngeal branch, and communicates with the pharyngeal branch of the ninth and with the superior laryngeal and hypoglossal nerves. The =inferior pharyngeal branch= is formed by the union of roots derived from the superior pharyngeal nerve and the ganglion nodosum, together with filaments from the superior cervical ganglion. It crosses the side of the pharynx behind the superior pharyngeal and ramifies on the posterior constrictor of the pharynx and the origin of the œsophagus. It contributes filaments to the pharyngeal plexus, communicates with the recurrent nerve, and supplies twigs to the thyroid gland. The =pharyngeal plexus= is formed on the lateral surface of the pharynx by branches of the pharyngeal nerves and the communications above described; branches from it innervate the muscles and mucous membrane of the pharynx. The =superior laryngeal nerve= arises from the ganglion nodosum and descends over the side of the pharynx, crossing beneath the carotid artery and the superior laryngeal nerve. It passes through the thyroid notch and ramifies in the mucous membrane of the larynx. It communicates with the superior cervical ganglion and the superior pharyngeal branch of the vagus, and gives twigs to the hyo-pharyngeus muscle. Immediately after its entrance into the larynx it gives off a large branch which, instead of uniting with the recurrent as in the other animals, has a peculiar arrangement. It runs back near the dorsal border of the thyroid cartilage, gives a branch to the crico-arytenoideus posterior as it passes over that muscle, and continues along the trachea internal to the recurrent nerve. At the thoracic inlet it communicates with the inferior cervical ganglion and continues backward to unite with the vagus at or near the point of origin of the recurrent nerve. Its collateral filaments supply the trachea and concur with the inferior pharyngeal branch in forming a plexus on the cervical part of the œsophagus which innervates that tube.[216] The =depressor nerve= is a very delicate filament which arises usually from the superior laryngeal nerve and accompanies the vago-sympathetic trunk to the thorax; its fibers reach the heart through the cardiac nerves. The =pulmonary= and =posterior œsophageal plexuses= are highly developed.

The =hypoglossal nerve= gives off a long branch (R. descendens) which runs downward and backward across the pharynx and larynx, communicates with the ventral branch of the first cervical nerve, and supplies the thyro-hyoid, sterno-hyoid, and sterno-thyroid muscles.

The =spinal nerves= number thirty-six or thirty-seven on either side, and comprise eight cervical, thirteen thoracic, seven lumbar, and five or six coccygeal.

The =brachial plexus= (Fig. 487) is derived from the ventral branches of the last four cervical and first thoracic nerves; the root supplied by the fifth cervical nerve is very small. The roots unite at the lower border of the scalenus. The more important special features in the arrangement of the nerves which emanate from the plexus are as follows:

The =musculo-cutaneous nerve= passes between the coraco-brachialis and the brachial artery and descends in the arm in front of the artery. At the shoulder joint it gives off branches to the biceps and coraco-brachialis, and in the lower third of the arm is connected with the median nerve by an oblique branch. It terminates near the elbow by dividing into a branch for the brachialis and a small cutaneous nerve which passes down over the inner face of the elbow and, inclining a little forward, descends over the deep fascia of the forearm to the carpus.

The =radial nerve= descends behind the ulnar nerve, gives branches to the extensors of the elbow, dips in between the internal head of the triceps and the accessory head of the anconeus, winds around the arm, and divides between the brachialis and the external head of the triceps into two branches. The =deep branch= (R. profundus) supplies the extensor and supinator muscles on the forearm. The =superficial branch= (R. superficialis) emerges upon the flexor surface of the elbow and divides into two branches which terminate by supplying two =dorsal digital nerves= to each digit, except the fifth, which receives its outer dorsal nerve from the ulnar. The inner branch descends along the inner side of the cephalic vein to the carpus, where it divides into dorsal digital nerves for the first digit and the inner side of the second. The outer branch is much larger. It runs downward on the middle of the front of the forearm and supplies the remaining dorsal digital nerves except that to the outer side of the fifth digit.

The =ulnar nerve= is as large as or larger than the median, with which it is united for some distance. At the distal third of the arm it separates from the median and passes over the internal epicondyle of the humerus. At the proximal part of the forearm it gives off the =dorsal branch= (R. dorsalis), which supplies cutaneous twigs to the dorso-lateral surface of the lower part of the forearm and carpus and terminates as the =external dorsal digital nerve= of the fifth digit. Descending under cover of the flexor carpi ulnaris, the ulnar nerve inclines inward under the tendon of insertion of that muscle and divides into superficial and deep branches. The =superficial branch= (R. superficialis) runs downward along the outer border of the flexor tendons, gives off the =external volar digital nerve of the fifth digit= (N. dig. vol. lat. dig. V), and a branch (N. met. vol. IV) which descends in the space between the fourth and fifth metacarpal bones and unites with the deep branch. The =deep branch= (R. profundus) descends in the carpal canal and divides under the deep flexor tendon into its terminal branches. The smaller of these supply the volar metacarpal muscles. The larger terminals are the three =volar common digital nerves= (Nn. dig. vol. comm. II, III, IV), which descend along the second, third, and fourth intermetacarpal spaces, subdivide, and concur with the volar metacarpal branches of the median nerve in forming the =volar proper digital nerves= (Nn. dig. vol. proprii).

The =median nerve= descends behind the brachial artery, passes over the internal epicondyle of the humerus, then under the pronator teres, and continues in the forearm under cover of the flexor carpi radialis. It gives branches below the elbow to the flexor and pronator muscles, and lower down a palmar branch to the skin on the inner and posterior aspect of the carpus, and terminates between the superficial and deep flexor tendons by dividing into three =volar metacarpal nerves= (N. met. vol. I, II, III). These descend in the first, second, and third intermetacarpal spaces and unite with the volar common digital nerves in forming volar proper digital nerves. The arrangement of the digital nerves is indicated in the annexed schematic figures.

FIG. 547.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF DOG, ANTERIOR
VIEW (SCHEMATIC).

_p_, Dorsal proper digital nerves.
]

FIG. 548.—NERVES OF DISTAL PART OF RIGHT FORE LIMB OF DOG, POSTERIOR
VIEW (SCHEMATIC).

_p_, Volar proper digital nerves.
]

The =lumbo-sacral plexus= is formed from the ventral branches of the last five lumbar and first sacral nerves. The more important special facts in regard to the nerves of the pelvic limb are as follows:

The =saphenous nerve= (Fig. 492) is relatively large, and, in fact, might be considered as the continuation of the femoral nerve. It descends at first in front of the femoral artery, then passes over the lower part of the inner surface of the thigh with the saphenous artery, continues down the leg with the dorsal branch of that vessel, and anastomoses with branches of the superficial peroneal nerve. It supplies cutaneous twigs from the stifle to the metatarsus.

The =peroneal nerve= separates from the tibial above the origin of the gastrocnemius, runs downward and forward across the outer head of that muscle, passes between the deep flexor of the digit and the peroneus longus, and divides into superficial and deep branches. The =superficial peroneal nerve= descends the leg along the peroneus tertius and longus, gives off a =dorsal branch= to the first digit (N. cutaneus dorsalis medialis), and divides at the proximal part of the metatarsus into three =dorsal common digital nerves= (Nn. dig. pedis dors. comm. II, III, IV). These descend with the superficial dorsal metatarsal arteries and concur with branches of the deep peroneal nerve in forming =dorsal proper digital nerves= (Nn. dig. ped. prop. dorsales). The =deep peroneal nerve= passes down the leg with the anterior tibial vessels. It gives twigs to the hock joint and the extensor brevis muscle, and divides into three =dorsal metatarsal nerves= (Nn. met. dors. II, III, IV); these descend with the corresponding arteries along the intermetatarsal spaces and unite with the dorsal common digital nerves in supplying the dorsal proper digital nerves.

FIG. 549.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF DOG, ANTERIOR
VIEW (SCHEMATIC).

_p_, Dorsal proper digital nerves.
]

FIG. 550.—NERVES OF DISTAL PART OF RIGHT HIND LIMB OF DOG, POSTERIOR
VIEW (SCHEMATIC).

_p_, Plantar proper digital nerves.
]

The =tibial nerve= divides at the tarsus into two plantar branches. The =internal plantar nerve= descends along the inner border of the superficial flexor tendon and divides near the middle of the metatarsus into two branches. Of these, the internal branch constitutes the =first plantar common digital nerve= (N. dig. comm. plant. I). The outer branch descends on the superficial flexor tendon and divides into three =plantar metatarsal nerves= (Nn. met. plant. II, III, IV), which unite with the plantar common digital nerves. The =external plantar nerve= passes down between the flexor tendons, sends twigs to the muscles on the plantar surface of the metatarsus, and divides into three =plantar common digital nerves= (Nn. dig. comm, plant. II, III, IV). These descend with the deep plantar metatarsal arteries in the second, third, and fourth intermetatarsal spaces, receive the plantar metatarsal nerves, and divide into =plantar proper digital nerves=, which pass down the opposed surfaces of the second to the fifth digits.

The =sympathetic system= presents few special features worthy of mention. The =superior cervical ganglion= lies above the ganglion nodosum close to the cranial base and internal to the origin of the internal carotid and occipital arteries. It forms connections directly or through the carotid plexus with the last seven cranial and first cervical nerves. The =cervical trunk= unites with the vagus so intimately as to form a vago-sympathetic trunk. There may be a small =middle cervical ganglion= in front of the inferior cervical ganglion. The latter is usually distinct from the first thoracic ganglion, which is stellate and lies on the longus colli opposite the second rib and intercostal space.

ÆSTHESIOLOGY

THE SENSE ORGANS AND COMMON INTEGUMENT

The =organs of the senses= (Organa sensuum) receive external stimuli and conduct impulses to the brain which result in sensations of sight, hearing, taste, smell, and touch. They consist essentially of specially differentiated cells, the neuro-epithelium, and a conduction path which is simple in the more generalized sense organs, elaborate in those which are highly specialized—the eye and the ear.

THE SENSE ORGANS AND SKIN OF THE HORSE

THE EYE

The =eye= or =organ of vision= (Organon visus) in the broader sense of the term comprises the eyeball or globe of the eye, the optic nerve, and certain accessory organs associated therewith. The accessory organs (Organa oculi accessoria) are the orbital fasciæ and muscles, the eyelids and conjunctiva, and the lacrimal apparatus. These structures will be considered in the order in which they may be most conveniently examined, taking the horse as a type. The bony walls of the orbit have been described in connection with the skull; the periorbita, a fibrous membrane which incloses the eyeball together with its muscles, vessels, and nerves, may be appropriately included in the account of the fasciæ.

FIG. 551.—LEFT EYE OF HORSE.

_9_, Zygomatic arch; _10_, supraorbital depression; _12_, supraorbital
process; _27_, facial crest. (After Ellenberger-Baum, Anat. für
Künstler.)
]

THE EYELIDS AND CONJUNCTIVA

The =eyelids=, upper and lower (Palpebra superior et inferior), are movable folds of integument situated in front of the eyeball. When closed, they cover the entrance to the orbit and the anterior surface of the eyeball. The upper lid is much more extensive and more movable than the lower one, and its free edge is more concave. The interval between the lids is termed the =palpebral fissure= (Rima palpebrarum). When the eye is closed, it is an oblique slit about two inches (ca. 5 cm.) in length; when open, it is biconvex in outline. The ends of the fissure are the =angles= or =canthi=, and are distinguished as =internal= or =nasal=, and =external= or =temporal= (Angulus oculi medialis, lateralis). The external angle is rounded when the eye is open, but the internal angle is narrowed to form a ᑐ-shaped bay or recess, termed the =lacrimal lake= (Lacus lacrimalis). In this there is a rounded, pigmented prominence known as the =lacrimal caruncle= (Caruncula lacrimalis); it is about the size of a small pea and is covered with modified skin, connected with that of the inner commissure, from which project a number of hairs provided with sebaceous glands. The lids unite on either side and form the =commissures=, internal and external (Commissura palpebrarum medialis, lateralis). The =anterior surface= of the lids (Facies anterior palpebrarum) is convex and presents a variable number of long tactile hairs in addition to the ordinary but very short hairs. A considerable number of tactile hairs are scattered over the lower part of the lower lid, but on the upper lid they are very scanty. The infrapalpebral depression (Sulcus infrapalpebralis) indicates somewhat indistinctly the limit of the lower lid. The upper lid is marked by two furrows when raised. The =posterior surface= (Facies posterior palpebrarum) is adapted to the free surface of the eyeball and is covered by the palpebral conjunctiva. The free border of the lid is smooth and usually black. It has a well defined posterior margin (Limbus palpebralis posterior), along which the ducts of the tarsal glands open. The anterior margin (Limbus palpebralis anterior) bears stiff hairs termed the cilia or eyelashes. On the upper lid the cilia are long and numerous except at its inner third, where they are very small or absent. On the lower lid the cilia are often scarcely distinguishable from the ordinary hairs; in other cases they may be clearly seen except near the external canthus, and are much finer and shorter than those of the upper lid. The edge of each lid is pierced near the internal angle by a minute, slit-like opening, the =punctum lacrimalis=, which is the entrance to the lacrimal duct or canaliculus.

FIG. 552.—VERTICAL, SECTION OF ANTERIOR PART OF EYE OF HORSE, WITH
LIDS HALF CLOSED.

_1_, Tarsal gland of upper lid; _2_, palpebral conjunctiva; _3_,
fornix conjunctivæ; _4_, levator palpebræ superioris; _5_,
orbicularis oculi; _6_, cornea; _7_, anterior chamber; _8_, iris;
_9_, _9′_, granula iridis; _10_, posterior chamber; _11_, ciliary
process; _12_, ciliary muscle; _13_, ciliary zone or suspensory lig.
of lens; _14_, chorioid; _15_, sclera; _16_, lens; _17_, root of
tactile hair. (After Bayer, Augenheilkunde.)
]

=Structure of the Eyelids.=—The skin of the eyelids is thin and freely movable, except near the free edge, where it is more firmly attached. The underlying subcutaneous tissue is destitute of fat. The =muscular layer= consists chiefly of the elliptical bundles of the orbicularis oculi, with which are associated fibers of the corrugator supercilii in the upper lid and the malaris in the lower lid. At the inner side there is a fibrous band, the =palpebral ligament=, which is attached to the lacrimal tubercle and furnishes origin to some fibers of the orbicularis. At the inner commissure a bundle detached from the orbicularis passes inward behind the lacrimal sac, and is known as the pars lacrimalis or Horner’s muscle. At the outer side an indistinct palpebral raphé occurs where fibers of the orbicularis decussate. The =fibrous layer= is thicker and denser along the free edge of the lid, forming here the =tarsus=. The tarsus furnishes insertion to a layer of unstriped muscle known as the tarsal muscle (of Müller). The =tarsal glands= (Glandulæ tarsales) are partly embedded in the deep face of the tarsus, and are visible when the lid is everted if the conjunctiva is not too strongly pigmented. They are arranged in a linear series, close together, and with their long axes perpendicular to the free edge of each lid. In the upper lid they number forty-five to fifty; in the lower, thirty to thirty-five. Each consists of a tubular duct beset with numerous alveoli, in which a fatty substance, the =palpebral sebum=, is secreted. The =palpebral conjunctiva= lines the posterior surface of the eyelids.

FIG. 553.—PIECE OF UPPER EYELID, INNER SURFACE.

_1_, Cilia; _2_, limbus palp. posterior; _3_, tarsal glands. (After
Ellenberger, in Leisering’s Atlas.)
]

FIG. 554.—CARTILAGE OF THIRD EYELID OF HORSE, CONVEX SURFACE.

_1_, Gland; _2_, fat surrounding deep part of cartilage. (After
Ellenberger, in Leisering’s Atlas.)
]

The =conjunctiva= is the mucous membrane which lines the lids as =palpebral conjunctiva= (C. palpebrarum) and is reflected upon the anterior part of the eyeball as =bulbar conjunctiva= (C. bulbi); the line of reflection is termed the =fornix conjunctivæ=. The palpebral part is closely adherent to the tarsus, but is loosely attached further back. It is papillated and is covered with stratified cylindrical epithelium in which many goblet cells are present. In the fornix and its vicinity there are tubular glands (of Henle). Near the inner angle there are numerous lymph nodules. The conjunctiva of the outer part of the upper lid is pierced near the fornix by the orifices of the excretory ducts of the lacrimal gland. The bulbar conjunctiva is loosely attached to the anterior part of the sclera and is pigmented in the vicinity of the corneo-scleral junction. On the cornea it is represented by a stratified epithelium.

When the lids are in apposition, the conjunctiva (including the
epithelium of the cornea) forms a complete =conjunctival sac=, as the
capillary space between the lids and the eyeball is termed.

The =third eyelid= (Palpebra tertia) is situated at the inner angle of the eye (Figs. 551, 556). It consists of a semilunar fold of the conjunctiva, known as the =membrana nictitans=, which covers and partly incloses a curved plate of hyaline cartilage. Its marginal part is thin and usually more or less pigmented. The cartilage has an elongated quadrilateral outline. The part of it which lies in the membrana is wide and thin. The deep part is narrower and thicker and is embedded in fat at the inner side of the eyeball. Numerous minute lymph nodules occur in the membrana nictitans, and the deep part of the cartilage is surrounded by a gland which resembles the lacrimal gland in structure (Glandula superficialis palpebæ tertiæ).

Ordinarily the third eyelid extends very little over the inner end of
the cornea, but when the eyeball is strongly retracted (as in tetanus)
the membrana is protruded over it so as to measure about an inch (ca.
2 to 3 cm.) in its middle. This effect results from the pressure of
the eyeball and its muscles on the fat which surrounds the deep part
of the cartilage.

=Vessels and Nerves.=—The =arteries= which chiefly supply the eyelids and conjunctiva are branches of the ophthalmic and facial arteries, and the blood is drained away by corresponding veins. The =sensory nerves= are branches of the ophthalmic and maxillary divisions of the trigeminus. The =motor nerves= to the orbicularis oculi, corrugator supercilii, and malaris come from the facial nerve, the levator palpebræ superioris is innervated by the oculomotor, and the unstriped muscle of the lids by the sympathetic.

THE LACRIMAL APPARATUS

The =lacrimal apparatus= (Apparatus lacrimalis) comprises: (1) the lacrimal gland, which secretes the clear lacrimal fluid; (2) the excretory ducts of the gland; (3) the two lacrimal ducts or canaliculi, lacrimal sac, and naso-lacrimal duct, which receive the fluid and convey it to the nostril.

The =lacrimal gland= (Glandula lacrimalis) is situated between the supraorbital process and the dorso-lateral surface of the eyeball (Fig. 438). It is flattened, oval in outline, and measures about two inches (ca. 5 cm.) transversely and an inch or more (2.5 to 3 cm.) in the sagittal direction. Its upper face is convex and is related to the concave lower surface of the supraorbital process. The lower face is concave in adaptation to the eyeball, from which it is separated by the periorbita. The =excretory ducts= (Ductuli excretorii) are very small and are twelve to sixteen in number; they open into the outer part of the conjunctival sac along a line a little in front of the fornix conjunctivæ superior. In appearance and structure the gland resembles the parotid. It receives its blood-supply chiefly from the lacrimal artery. The sensory nerve is the lacrimal, and the secretory fibers are derived from the sympathetic.

The =puncta lacrimalia= are the entrances to the two lacrimal ducts. Each is a fine slit-like opening (about 2 mm. long), situated close behind the free edge of the lid and about a third of an inch (ca. 8 mm.) from the internal canthus. The =lacrimal ducts= (Ductus lacrimales), upper and lower, begin at the puncta and converge at the inner commissure to open into the =lacrimal sac=. The latter (Saccus lacrimalis) may be regarded as the dilated origin of the naso-lacrimal duct. It occupies the funnel-like origin of the bony lacrimal canal, and leads to the =naso-lacrimal duct= (Ductus naso-lacrimalis), which passes forward and a little downward along the outer wall of the frontal sinus and the nasal cavity and opens near the lower commissure of the nostril. Its length is about ten to twelve inches (ca. 25 to 30 cm.). In the first part of its course it is inclosed in the osseous lacrimal canal; further forward it lies in the lacrimal groove of the maxilla, covered at first by a plate of cartilage and then by the mucous membrane of the middle meatus. The terminal part lies in the inferior turbinal fold and opens on the skin of the floor of the nostril near the transition to mucous membrane. Accessory openings may occur a little further back.

The first part of the duct, about 6 to 7 mm. in diameter, extends in a
gentle curve, convex dorsally, from the internal commissure toward a
point just above the level of the infraorbital foramen. The second
part (isthmus) is narrower (ca. 3 to 4 mm.); it extends forward and a
little ventrally about to a transverse plane through the first cheek
tooth and lies in the groove above the inferior turbinal crest. Beyond
this the duct inclines upward and widens very considerably, crosses
the nasal process of the premaxilla obliquely, and contracts at its
termination. The mucous membrane may present valvular folds, the most
distinct of which is situated at the origin.

THE PERIORBITA

The =periorbita= or ocular sheath is a conical fibrous membrane which incloses the eyeball with its muscles, vessels, nerves, etc. Its apex is attached around the optic and lacerated foramina, and its base is in part attached to the bony rim of the orbit, in part continuous with the fibrous layer of the lids. Its inner part, which is in contact with the orbital wall, is thin; incorporated with it beneath the root of the supraorbital process is the bar of cartilage around which the superior oblique muscle is reflected. The outer part is thicker, and is strengthened by an elastic band which is attached to the pterygoid crest and furnishes origin to the thin unstriped orbital muscle. A quantity of fat (Corpus adiposum extraorbitale) lies about the periorbita, and within it is the intraorbital adipose tissue (Corpus adiposum intraorbitale) which fills the interstices between the eyeball, muscles, etc.

THE ORBITAL FASCIÆ AND OCULAR MUSCLES (Figs. 438, 439, 555)

The straight muscles of the eyeball and the oblique muscles in part are inclosed in fibrous sheaths (Fasciæ musculares), formed by superficial and deep layers of fascia, which are united by intermuscular septa in the interstices between the muscles. The =superficial fascia= is thin; it blends in front with the fibrous layer of the eyelids and is attached behind around the optic foramen. The =deep fascia= consists anteriorly of two layers, one of which is continuous with the fibrous tissue of the lids, while the other is attached at the corneo-scleral junction.

The posterior part of the eyeball is covered by the =bulbar fascia= or capsule of Tenon (Fascia bulbi), so that between them a lymph space (Spatium interfasciale) is inclosed which communicates with the subdural space along the course of the optic nerve.

The =levator palpebræ superioris= muscle[217] is a thin band about half an inch in width which lies above the rectus superior. It is narrow at its origin above and behind the ethmoidal foramen and ends by an expanded tendon in the upper lid. Its action is to raise the upper lid.

The =recti oculi= (Mm. recti bulbi) are four in number and are designated according to their positions as =superior=, =inferior=, =internal=, and =external=. They are all band-like, arise close together around the optic foramen, and diverge as they pass forward to the eyeball. On reaching the latter they end in thin tendons which are inserted into the sclera in front of the equator of the eyeball.

The =retractor oculi= (M. retractor bulbi) surrounds the optic nerve, and is incompletely divided into four parts which alternate with the recti. They arise around the optic nerve and are inserted into the sclera behind the recti.

The =obliquus oculi superior= (M. obliquus bulbi dorsalis s. superior) is the longest and narrowest of the ocular muscles. It arises near the ethmoidal foramen and passes forward internal to the rectus internus. Under the root of the supraorbital process it is reflected almost at a right angle around a cartilaginous pulley (trochlea), which is attached to the anterior part of the inner wall of the orbit, a bursa being interposed here. The muscle is then directed outward and somewhat forward, and ends in a thin tendon which passes between the rectus superior and the eyeball, and is inserted into the sclera between the superior and external recti, about half an inch behind the margin of the cornea.

The =obliquus oculi inferior= (M. obliquus bulbi ventralis s. inferior) is wide and much shorter than the recti. It arises from the inner wall of the orbit in the small depression (Fossa muscularis) behind the lacrimal fossa. It curves around the rectus inferior and is inserted into the sclera near and partly beneath the rectus externus.

=Actions.=—The superior and inferior recti rotate the eyeball about a transverse axis, moving the vertex of the cornea upward and downward respectively. Similarly the internal and external recti rotate the eyeball about a vertical axis, turning the vertex of the cornea inward and outward respectively. The oblique muscles rotate the eyeball about a longitudinal axis; the superior oblique raises the outer end of the pupil, while the inferior oblique lowers it. The retractor as a whole draws the eyeball backward, and its parts may separately reinforce the corresponding recti. Also the four recti acting together will retract the eyeball.

FIG. 555.—VERTICAL, AXIAL SECTION OF ORBIT OF HORSE.

_a_, Eyelids; _b_, bulbar fascia (Tenon’s capsule); _c_, _c′_,
retractor bulbi; _d_, rectus oculi inferior; _e_, obliquus oculi
inferior (in cross-section); _f_, rectus oculi superior; _g_,
levator palpebræ superioris; _h_, obliquus oculi superior (in
cross-section); _i_, lacrimal gland; _k_, _k′_, periorbita; _l_,
superficial fascia; _m_, deep fascia; _n_, skin; _o_, retrobulbar
fat; _p_, extraorbital fat; _q_, temporalis muscle; _r_,
supraorbital process; _s_, cranial wall; _1_, cornea; _2_, sclera;
_3_, chorioidea; _4_, ciliary muscle; _5_, iris; _6_, granula
iridis; _7_, retina; _7′_, optic papilla; _8_, optic nerve; _9_,
crystalline lens; _10_, capsule of lens; _11_, ciliary zone; _12_,
posterior chamber; _13_, anterior chamber; _14_, conjunctiva bulbi;
_15_, vitreous body. (After Ellenberger, in Leisering’s Atlas.)
]

The actual movements of the eyeball are by no means so simple as might
be inferred from the foregoing general statements. Practically all
movements are produced by the coördinated actions of several muscles,
involving combinations which are quite complex and difficult to
analyze accurately. Further complication is caused by the fact that
the recti are not inserted at equal distances from the equator and the
axes of rotation of the oblique muscles do not correspond to the
longitudinal axis of the eyeball.

=Nerve-supply.=—The oculomotor nerve supplies the foregoing muscles, with the exception of the rectus externus and obliquus superior, which are innervated by the abducens and trochlearis respectively.

THE EYEBALL

The =eyeball= (Bulbus oculi) is situated in the anterior part of the orbital cavity, protected in front by the eyelids and conjunctiva, in its middle by the complete orbital ring, and related behind to the bulbar fascia, fat, and ocular muscles.

It has the form approximately of an oblate spheroid, but is composed of the segments of two spheres of different sizes. The anterior, transparent, corneal segment has a radius of curvature of about 17 mm., and the posterior, opaque, scleral segment a radius of about 25 mm. The anterior segment therefore projects more strongly, and the junction of the two segments is marked externally by a broad, shallow groove, the sulcus scleræ. The central points of the anterior and posterior curvatures of the eyeball are termed respectively the =anterior= and =posterior poles= (Polus anterior, posterior), and the line connecting the poles is the =optic axis= (Axis oculi externa). The angle of divergence of the optic axes is about 137 degrees. The =equator= (Æquator) is an imaginary line drawn around the eyeball midway between its poles, and =meridians= (Meridiani) are lines drawn around it through the poles.

The average transverse diameter of the eyeball is about 5 cm., the
vertical about 4.5 cm., and the axial about 4.25 cm. The distance from
the anterior pole to the point of entrance of the optic nerve is about
3 cm.

FIG. 556.—LEFT EYEBALL OF HORSE, _in situ_, AFTER REMOVAL OF UPPER AND
LOWER LIDS.

_9_, Zygomatic arch; _12_, supraorbital process; _19_, orbital fat;
_27_, facial crest. (After Ellenberger-Baum, Anat. für Künstler.)
]

The eyeball consists of three concentric tunics or coats, within which three refractive media are inclosed.

THE FIBROUS TUNIC

The =fibrous tunic= (Tunica fibrosa oculi) is the external coat and is composed of a posterior opaque part, the sclera, and a transparent anterior part, the cornea.

The =sclera=, popularly termed the “white of the eye,” is a dense fibrous membrane which forms about four-fifths of the fibrous tunic. Thickest in the vicinity of the posterior pole (ca. 2 mm.), it thins at the equator (ca. 0.4 mm.), and increases in thickness toward the junction with the cornea (ca. 1.3 mm.). It is in general white, but may have a bluish tinge in its thinnest parts. Its external surface furnishes insertion to the ocular muscles and is covered by the conjunctiva scleræ in its anterior part. The episcleral tissue, which is richly supplied with vessels and nerves, attaches the conjunctiva to the sclera; it is abundant and loosely meshed except at the junction with the cornea. The inner surface is attached to the chorioid coat by a layer of delicate, pigmented, connective tissue, the =lamina fusca=. The anterior border is oval, the long axis being transverse, and is continuous with the cornea. The transition from the opaque scleral tissue to the transparent corneal substance occurs in such manner that the sclera appears to form a groove (Rima cornealis), into which the cornea fits somewhat as a watch-glass in the case. Near the corneo-scleral junction there is a circular venous plexus, the =plexus= s. =sinus venosus scleræ=, formerly called the canal of Schlemm. The optic nerve passes through the posterior part of the sclera a little below and external to the posterior pole. The opening for the nerve is crossed by interlacing fibrous strands, forming the =lamina cribrosa scleræ=.

The sclera consists of interlacing bundles of white fibrous tissue, associated with which there are a few elastic fibers. The bundles are arranged chiefly in meridional and equatorial layers. The very limited blood-supply is derived from the ciliary arteries, and the veins open into the venæ vorticosæ and ciliary veins. The lymphatics are represented by intercommunicating cell spaces. The nerves are derived from the ciliary nerves.

FIG. 557.—VERTICAL SECTION OF EYEBALL OF HORSE, ABOUT ³⁄₂.

The contour of the crystalline lens is dotted.
]

The =cornea= forms the anterior fifth of the fibrous tunic. It is transparent, colorless, and non-vascular. Viewed from in front it is oval in outline, the long axis being transverse and the broad end internal; it appears more nearly circular when viewed from behind. Its =anterior surface= (Facies anterior) is convex and is more strongly curved than the sclera; its central part is termed the =vertex corneæ=. The =posterior surface= (Facies posterior) is concave; it forms the anterior boundary of the anterior chamber, and is in contact with the aqueous humor. The margin (limbus corneæ) joins the sclera; the latter overlaps the cornea more in front than behind, and more above and below than at the sides, thus explaining the difference in outline of the two surfaces. The cornea is thinnest at the vertex.

The cornea consists, from before backward, of the following layers: (1) The =epithelium corneæ= is continuous with that of the conjunctiva scleræ, and is of the stratified squamous type. (2) The =lamina limitans anterior= is merely a condensation of the next layer. (3) The =substantia propria= forms the bulk of the cornea and is composed of interlacing bundles of connective tissue, arranged in part in lamellæ disposed parallel with the surface. In the amorphous cement substance between the lamellæ are flattened connective-tissue cells, the corneal corpuscles. These have branching processes which unite with those of other cells, thus forming a protoplasmic network.[218] (4) The =lamina elastica posterior=, also termed the membrane of Descemet or Demours, is a thin and practically homogeneous membrane which is less intimately attached to the substantia propria than the anterior lamina. It is clear, glistening, and elastic. At the periphery the lamina divides into three sets of fibers. The anterior fibers join the sclera, the middle give attachment to the ciliary muscle, while the posterior pass into the iris and form the =ligamentum pectinatum iridis=. (5) The =endothelium= (cameræ anterioris) consists of a layer of flattened polygonal cells, and is reflected on to the anterior surface of the iris.

The cornea is without =blood-vessels= except at its periphery, where the terminal twigs of the vessels of the sclera and conjunctiva terminate in loops. The =nerves= are derived from the ciliary nerves. They form a plexus around the periphery (Plexus annularis), from which fibers pass into the substantia propria, become non-medullated, and form the fundamental or stroma plexus. From this perforating branches pass through the anterior limiting layer and form a subepithelial plexus, from which filaments ascend between the epithelial cells. Other branches from the plexuses in the substantia propria end as fibrils which are in close relation with the corneal corpuscles.

FIG. 558.—TAPETUM OF HORSE.

_a_, Optic papilla; _b_, lower border of tapetum. (After Ellenberger,
in Leisering’s Atlas.)
]

THE VASCULAR TUNIC

The =vascular tunic= (Tunica vasculosa oculi) lies internal to the fibrous coat; it comprises three parts—the chorioid, the ciliary body, and the iris.

1. The =chorioid= (Chorioidea) is a thin membrane which lies between the sclera and retina. It is in general rather loosely attached to the sclera by the lamina fusca, but is intimately adherent at the point of entrance of the optic nerve and less closely in places where the ciliary vessels and nerves pass through. The inner surface is in contact with the layer of pigmented cells of the retina, which adhere so closely to the chorioid that they were formerly regarded as a part of the latter. The general color of the chorioid is dark brown, but an extensive semilunar area a little above the level of the optic papilla has a remarkable metallic luster, and is termed the =tapetum= of the chorioid (Tapetum chorioideæ). The appearance here varies in different individuals, but the prevailing colors in most cases are iridescent blue and green in various nuances shading into yellow. Posteriorly the chorioid is perforated by the optic nerve, and anteriorly it is continuous with the ciliary body.

The chorioid consists of four layers, which from without inward are as follows: (1) The =lamina suprachorioidea= consists of interlacing fine lamellæ of fibrous tissue, each containing a network of elastic tissue. Among these are large-branched, pigmented, connective-tissue cells. The spaces between the lamellæ are lined with endothelium, and form a system of lymph-clefts which together form the perichorioid space (Spatium perichorioideale). (2) The =lamina vasculosa= is the outer part of the proper tissue of the chorioid. It contains the larger blood-vessels, which are supported by connective-tissue and elastic fibers. (3) The =lamina choriocapillaris= consists of an extremely rich network of capillaries embedded in an almost homogeneous matrix. Between it and the lamina vasculosa is a layer of fibro-elastic tissue, the =tapetum fibrosum=, which causes the metallic luster mentioned above. (4) The =lamina basalis= is very thin and transparent. It is composed of an inner homogeneous part and an outer elastic part.

2. The =ciliary body= (Corpus ciliare), the middle part of the vascular coat, connects the chorioid with the periphery of the iris. In meridional section it has the form of a narrow triangle, the base of which is next to the iris. On its inner side are the ciliary processes and on its outer side the ciliary muscle. It consists of three parts—the ciliary ring, ciliary processes, and ciliary muscle. The =ciliary ring= (Orbicularis ciliaris) is the posterior zone, which is distinguished from the chorioid mainly by its greater thickness and the absence of the choriocapillaris. Its inner face presents numerous fine meridional ridges, by the union of which the ciliary processes are formed. The =ciliary processes= (Processus ciliares), more than a hundred in number, form a circle of radial folds which surround the lens and furnish attachment to the zonula ciliaris or suspensory ligament of the latter. They are small at their origin on the ciliary ring and become much thicker and higher toward their inner ends. The width of the circle formed by them is narrower at the inner side than elsewhere. Their bases extend forward to the periphery of the iris, and their inner ends are close to the margin of the lens. They bear numerous secondary folds. Their inner surface is covered by a continuation of the lamina basalis of the chorioid, on which there are two layers of epithelial cells which constitute the pars ciliaris retinæ. They consist of a rich network of tortuous vessels supported in pigmented connective tissue. The =ciliary muscle= (M. ciliaris) (Figs. 552, 555, 559) constitutes the outer part of the ciliary body, and lies between the sclera and the ciliary processes. It forms a circular band of unstriped muscle, the fibers of which are for the most part directed meridionally. They arise from the inner surface of the sclera and from the ligamentum pectinatum iridis close to the corneo-scleral junction, and run backward along the sclera to be inserted into the ciliary processes and ring. When the muscle contracts, it pulls the processes and ring forward, thus slackening the ciliary zone or suspensory ligament of the lens, and allowing the latter to become more convex. This is the mechanism of accommodation for near objects.

FIG. 559.—VASCULAR TUNIC OF EYEBALL OF HORSE, FRONT VIEW.

The cornea is removed and the sclera is reflected in flaps. _1_,
Sclera; _1′_, lamina fusca; _2_, chorioidea; _2′_, ciliary veins;
_3_, ciliary muscle; _4_, iris; _5_, _5′_, granula iridis; _6_,
pupil, through which the lens is visible. (After Ellenberger, in
Leisering’s Atlas.)
]

In man the muscle has the form of a prismatic ring which is triangular
in meridional section, the base being directed toward the periphery of
the iris. It consists chiefly of meridional fibers, but a ring of
circular fibers forms the inner angle of its base. In the horse the
muscle is much less developed, and has the form of a flat band; it
does not contain circular fibers, but the arrangement is rendered more
or less plexiform by the existence of oblique and equatorial fibers.

3. The =iris= (Figs. 552, 557, 559) is a muscular diaphragm placed in front of the lens, and is visible through the cornea. It is pierced centrally by an elliptical opening, the =pupil= (Pupilla), which varies in size during life and determines the amount of light admitted to act on the retina. In strong light the vertical diameter of the pupil is very short, but the opening is almost circular when the pupil is fully dilated. The =ciliary border= (Margo ciliaris) or periphery of the iris is continuous with the ciliary body and is connected with the corneo-scleral junction by the ligamentum pectinatum iridis. The ligament consists of strands of connective tissue which are attached externally to the corneo-scleral junction. Its bundles interlace and inclose spaces (Spatia anguli iridis) which are lined with endothelium and communicate with the anterior chamber. The =pupillary border= (Margo pupillaris) surrounds the pupil. Its upper part bears in its middle several black masses of variable size, termed the =granula iridis= or corpora nigra; similar, but much smaller, projections may be seen on the lower margin of the pupil. The =anterior surface= (Facies anterior) determines the color of the eye, which is dark brown usually. It is marked by delicate concentric lines (Plicæ iridis), which fade out near the pupil. The smooth, narrow, central part is termed the pupillary zone or annulus iridis minor, while the much broader plicated part is the ciliary zone or annulus iridis major. The =posterior surface= (Facies posterior) is usually black. It bears numerous fine radial lines except at the pupillary margin. Its central part is in contact with the anterior surface of the lens, but peripherally the two are separated by a narrow space termed the posterior chamber. The iris consists chiefly of the =stroma iridis=, a delicate framework of connective tissue, which supports numerous blood-vessels, and contains branched pigmented cells. The muscular tissue (unstriped) consists of a sphincter and a dilator of the pupil. The =sphincter pupillæ= lies in the posterior part around the pupil, with which the fibers are largely concentric. The =dilatator pupillæ= consists of fibers which radiate outward from the sphincter to the ciliary border. The anterior surface of the iris is covered by a continuation of the endothelium of the cornea. Beneath this is a condensation of the stroma, in which the cells are close together and are full of pigmented granules.[219] There appear to be minute clefts here by which the lymph-spaces of the stroma communicate with the anterior chamber. The posterior surface is covered by a pigmented epithelium, the stratum pigmenti iridis, which is a continuation of that of the ciliary body.

The =arteries= of the vascular tunic come from the ciliary branches of the ophthalmic artery. The arteries of the chorioidea are derived chiefly from the short posterior ciliary arteries. These perforate the sclera around the posterior pole, run forward in the lamina vasculosa, and form the rich capillary network of the choriocapillaris. The two long ciliary arteries perforate the sclera obliquely near the optic nerve; they run forward in the lamina suprachorioidea in the horizontal meridian, one on the inner, the other on the outer, side of the eyeball. On reaching the ciliary body each divides into diverging branches; the subdivisions of these unite with each other and with twigs of the anterior ciliary arteries to form a circular anastomosis, the =circulus arteriosus major=. From this branches go to the ciliary muscle and processes and to the iris. The two anterior ciliary arteries, dorsal and ventral, form an episcleral plexus around the corneo-scleral junction, and give off branches which perforate the sclera. These supply twigs to the ciliary muscle and recurrent branches to the chorioid, and assist in forming the circulus arteriosus major.