Chapter XII: Section III: The Nervous System (2)

*a.* The _medulla oblongata_ (Figs. 98, 102, and 103 _Mo_)--α. *External form.* The _medulla_ is limited behind by the origin of the first pair of spinal nerves, at which point a very faint constriction is sometimes found: it extends forwards as far as the _cerebellum_. It gradually widens as it passes forwards until just before it reaches its anterior limit, where it presents a shallow but sharp constriction. The *dorsal surface* is characterised by the presence of a deep, triangular fossa, the *fourth ventricle* (Fig. 98 _S.r_), (_ventriculus quartus_, Stieda; _sinus rhomboideus s. sinus triangularis_, Reissner; _fossa rhomboidalis_); the sides of the triangle are, however, not quite straight, but are slightly bent outwards just before they converge towards the posteriorly-directed apex; the base of the triangle is formed by the _cerebellum_. By careful examination, the ventricle is seen to be continued for a short distance under the _cerebellum_, where it opens into the Sylvian aqueduct. In the floor of the fourth ventricle is a well-marked median longitudinal fissure (_sulcus centralis_), (Fig. 98 _S_). Into the posterior part of the ventricle opens the central canal of the spinal cord. As the fourth ventricle is formed by the white matter passing to either side, and the simultaneous flattening of the grey matter, the floor of the fourth ventricle is composed of grey matter.

The fourth ventricle is closed in by a highly vascular membrane, the *choroid plexus* of the fourth ventricle (_plexus choroideus ventriculi quarti_, Reissner; _velum medullare posterius_). The blood-vessels of the plexus will be described together with the other vessels of the brain (p. 162). They are supported by a connective-tissue matrix, and the whole covered with flattened epithelium, which in the fourth ventricle is ciliated and often pigmented.

The *ventral surface* of the _medulla oblongata_ (Fig. 102 _Mo_) has a median ventral longitudinal fissure, a direct continuation of that of the cord; in the anterior part of the _medulla oblongata_ there is also to either side of this a lateral fissure, continued on to the _crura cerebri_; these fissures correspond to the positions of the two _rami posteriores_ of the internal carotid arteries; they are always well seen in microscopical sections. The _medulla oblongata_ is so intimately connected with the _pars commissuralis_ (pp. 149, 150) that the minute anatomy of the two is best described at the same time.

β. *Internal structure.* Examined by means of serial sections, the _medulla oblongata_ is seen to have, in comparison with the cord, an increased amount of grey matter; this is especially the case in its anterior part. The floor and inner parts of the walls of the ventricle are formed of grey matter, in which the largest-sized cells have disappeared, to be replaced by medium-sized cells. Traced from behind, the ventral horns of the cord are seen to increase in size and to be more widely separated until they form two isolated masses, while the dorsal horns gradually diminish; at the same time they are forced outwards and upwards, until they lie under the floor of the ventricle, and so extend to the _pars peduncularis_.

Dorsal view of brain of _Rana esculenta_.

_Ad_ Choroid plexus.
_C_ Cerebellum.
_f_ Groove between cerebral hemispheres and olfactory lobes.
_G_ Opening in the roof of the third ventricle.
_Gp_ Pineal body.
_Hc_ Cerebral hemispheres.
_I_ Olfactory nerve.
_Li_ Wall of fourth ventricle.
_L.ol_ Olfactory lobe.
_L.op_ Optic lobe.
_Mo_ Medulla oblongata.
_S_ Longitudinal fissure of the fourth ventricle.
_S.r_ Fourth ventricle.
_Tho_ Thalamencephalon.
]

*Grey matter.* The _substantia reticularis_ is not present, but the _septum medium_ extends forwards as far as the _pars peduncularis_. The central canal extends upwards at the expense of the tissue above it, and is here pear-shaped; at the same time the dorsal longitudinal fissure deepens until the two meet in the fourth ventricle; beyond this point one cannot speak of dorsal and ventral horns.

Transverse section through hinder end of _Medulla oblongata_ (magnified 30–80)[52], from Stieda.

[Footnote 52: In these diagrams, from Ludwig Stieda’s Studien über das centrale Nervensystem der Wirbelthiere, the outline of the diagram is magnified thirty times, while the details are magnified eighty times.]

_b_ Inferior commissure.
_f_ Dorsal horns.
_g_ Ventral horns.
_h_ Fourth ventricle.
_i_ Nucleus centralis.
_k_ Isolated mass of grey matter in which longitudinal
fibres of the pneumogastric nerve course.
]

Small nerve cells are irregularly distributed throughout the whole of the grey matter and cannot be grouped; the larger cells, on the other hand, are arranged in distinct groups which have special relations with the nerves arising from the part. Occasionally these groups are not so isolated as usual; in this case processes of one group can be traced into another group (Reissner). Of these groups the chief are:

1. The _nucleus centralis_ (upper inner group, Reissner), _nucleus medullae oblongatae_, Stieda (Fig. 99 _i_), is a group of cells found towards the hinder end of the _medulla oblongata_, on either side of and below the central canal; the group can be traced under the floor of the fourth ventricle to about its middle. The cells are rounded or spindle-shaped, the processes directed upwards, downwards, or outwards; their average size is 0·040–0·048 mm. long by 0·020 mm. broad.

2. The *auditory nucleus* (_nucleus acusticus_, Reissner, Stieda), (Fig. 100 _n_) is a large group of cells found in the wall of the fourth ventricle opposite the point of origin of the auditory nerve. The cells are rounded, pear-shaped, or of spindle form, and interspersed between the nerve fibres; these cells have an average length of 0·040 mm., and are about half as broad. The fibres of the auditory nerve radiate from their superficial origin in all directions through the grey matter towards these cells, and evidently communicate with them (Fig. 101 _p_). One small group (Fig. 101 _r_) passes to a lower level than the rest, and is regarded by Stieda as the true auditory centre. Köppen considers that the auditory nerve has a threefold origin: (1) from small cells on the median surface of the auditory area; (2) from the large cells between the above; (3) from a group of free nuclei on the dorsal surface of the auditory area.

Transverse section through the _Medulla oblongata_ at the point of origin of the abducens nerve, from Stieda. (Magnified 30 x 80.)

_h_ Fourth ventricle.
_m_ Abducens nerve.
_n_ Auditory nucleus.
_o_ Abducens nucleus.
]

3. The *trigeminal nucleus* (_nucleus trigeminus_), (Fig. 101 _q_) lies in part beneath the auditory nucleus but extends further forwards. It forms a rounded group of cells placed under the outer angle of the grey matter. The cells are somewhat crowded together, and are chiefly of an elongated spindle-form, with their processes directed obliquely downwards and outwards. The fibres of the trigeminal nerve separate into two groups; the upper group is best traced in a horizontal section, the fibres curving round to join the longitudinal fibres continued from the dorsal columns of the cord. The fibres of the lower, smaller group pass transversely inwards to the trigeminal nucleus. According to Reissner the latter fibres are motor, the former sensory. Probably other nerves are connected with the hinder part of this group.

4. The *abducens nucleus* (Fig. 100 _o_). From its superficial origin, the fibres of the abducens nerve may be traced vertically upwards to a small, rounded, grey mass; at this point the mass is somewhat isolated, but further forwards it may be traced as belonging to the central grey matter; it contains small spindle cells.

Transverse section of the _Medulla oblongata_, at the point of origin of the auditory nerve, from Stieda. (Magnified 30 x 80.)

_h_ Fourth ventricle.
_n_ Auditory nucleus.
_o_ Abducens nucleus.
_p_ Auditory nerve.
_p_′ Ganglion of auditory nerve.
_q_ Hinder portion of trigeminal nerve.
_r_ Bundle of fibres arising from trigeminal nucleus
and joining the auditory nerve.
]

5. The *pneumogastric nucleus*. The pneumogastric, with its numerous irregular roots, arises from the side of the _medulla oblongata_. The hindermost fibres can be traced as a small bundle, passing almost transversely through the white matter to the outer margin of the grey matter. The larger portion of the fibres is placed in front of these; part of this seems to be directly continuous with the longitudinal fibres of the white matter; a second part, however, can be traced from the surface transversely through the white matter to the grey matter. These latter fibres, together with those of the group first described, do not arise from the grey matter in this part of the _medulla oblongata_, but curve round and run backwards longitudinally through the grey matter, thus forming a rounded bundle of fibres (Fig. 99 _k_). Between these fibres are interspersed small nerve-cells and nuclei which disappear as the fibres approach the white matter. The vagus undoubtedly receives fibres from the grey matter throughout a long course, and again receives a large bundle just before leaving the grey matter. The more exact origin of the various fibres has not been traced.

6. The _nucleus magnus_ (Reissner and Stieda) is a very peculiar group of cells placed on either side, in the most anterior portion of the _pars commissuralis_, that is, immediately underneath the _valvula cerebelli_. The large cells are arranged in a transverse section in a single row so as to enclose a pear-shaped space on either side, which has its long axis directed from above, downwards and outwards, the narrower end being above. In longitudinal section the line of cells is seen to be open in front. The space enclosed by these cells is occupied by a granular ground-substance which contains only few nuclei. Bellonci is of opinion that these nuclei represent the _corpora quadrigemina posteriora_ of higher animals.

*White matter.* In the hinder part of the _medulla oblongata_ the arrangement of the white matter resembles that of the white matter of the spinal cord; further forwards the white matter of the dorsal surface commences to pass to either side, and ultimately it forms the outer part of the walls of the fourth ventricle. The fibres of the white matter of the ventral surface are unchanged in direction as they proceed forwards. The fibres of the anterior part of the medulla are thinner than those of the posterior portion (Stieda), according to Reissner they gradually thin as they pass forwards. The fibres are nearly all longitudinal, such transverse and oblique fibres as are present being chiefly in connection with the various nerve-roots and the commissures.

The _commissura superior_ is naturally lost in consequence of the opening of the central canal into the fourth ventricle; the _commissura inferior_ is increased in the anterior half and decreased in the posterior half of the _medulla oblongata_; in the latter the fibres become more and more oblique, and decussate very freely; ultimately they seem to be either continued as longitudinal fibres or to join the ganglia.

Near the _pars commissuralis_ is a transverse arched band of fibres, passing from the under surface of one half of the cord over the ventral longitudinal fissure through the _septum medium_ to the under surface of the opposite half; part of the fibres are continued upwards along the periphery to the _cerebellum_, part to the _nucleus magnus_. Vertical, straight, or slightly arched fibres are found in the walls of the fourth ventricle.

A section from the _medulla oblongata_ has a larger amount of *pigment* than a section from the spinal cord, and the anterior portion of the _medulla oblongata_ contains more than the posterior portion. The pigment is chiefly found in a curved line, placed in the lower and outer parts of the grey matter; the amount present varies in different specimens.

*b.* The _Cerebellum_ and _Valvula cerebelli_.

α. *External form.* The _cerebellum_ is a thin, semilunar plate, which projects between the optic lobes and the fourth ventricle, its base covering the most anterior part of the ventricle; the posterior surface possesses a very faint median fissure (Reissner). The _valvula cerebelli_ (_Velum medullare anterius_, Reissner) is the thin lamella which connects the anterior surface of the _cerebellum_ with the optic lobes.

β. *Minute structure.* By means of longitudinal, vertical sections, the posterior surface of the _cerebellum_ is seen to be covered with epithelium; in the lower part of the surface this is columnar or conical, above it is flattened: immediately beneath, that is in front of this is a finely granular layer, with very closely packed and granular nuclei. In front of these is a stratum of nerve-fibres forming the second layer of the cerebellum.

Still more anteriorly is the third layer of the cerebellum, an irregular double layer of large cells (Purkinje’s cells, Denissenko); the cells have an average length and breadth of 0.040 mm. and 0.015 mm respectively; they are pear-shaped or of spindle-form, and possess usually two well-marked processes, one passing into the layer behind, the other forwards into the anterior layer to be immediately described, while other less distinct processes radiate irregularly in all directions. The fourth and most anterior layer of the cerebellum is a thick stratum of nerve fibres with numerous nuclei (0.006 to 0.008 mm. diameter). The fibres are for the most part arranged transversely, but some course in various directions. These fibres underlie the flattened epithelium which covers the anterior surface of the cerebellum.

The fibres of the second layer course, for the most part, in an almost vertical plane; they connect the _cerebellum_ with the optic lobes (_processus cerebelli ad corpora bigemina_) and with other parts of the brain.

The fibres of the fourth layer receive numerous long processes from the large cells of Purkinje; they form a large commissural system, which can be followed ventrally on each side into the _pars commissuralis_. A part of the fibres ends here in the grey matter, a second portion enters the auditory area and forms a descending auditory root, a third part joins the lateral columns (in the _medulla oblongata_), and more anteriorly some join the ventral columns. The descending fibres from the _cerebellum_, together with the _fibrae arcuatae_ found in the ventral columns, indicate the presence of a _pons Varolii_. The fibres of this ventral commissure decussate only on its dorsal surface (Köppen).

The _Valvula cerebelli_ contains a few medullated fibres and the roots of the trochlear nerves; these pass from the _medulla oblongata_ into the _valvula cerebelli_, cross in the median line, and then proceed forwards as the trochlear nerves.

The *pigment* in the _pars commissuralis_ is arranged in a curved line similar to that found in the _medulla oblongata_, but the line is shortened at either extremity, and consequently does not extend into the _cerebellum_.

*c.* The *optic lobes* and _Crura cerebri_. (_Corpora geminata_ and _Pars peduncularis_, Reissner; _Lobus opticus_, Stieda; Vierhügel, Tiedemann; Vierhügel (Zweihügel) and _Pedunculi cerebri_, Schiess; Mesencephalon, Huxley.)

Ventral view of brain of _Rana esculenta_.

_Cho_ Optic chiasma.
_Hc_ Cerebral hemispheres.
_Hy_ Pituitary body.
_L.ol_ Olfactory lobe.
_L.ol^1_ Origin of olfactory nerve from
the cerebral hemisphere.
_Lt_ Lamina terminalis.
_Mo_ Medulla oblongata.
_To_ Optic tract.
_Tu.c_ Tuber cinereum.
_I_ 1st }
_I^1_ 2nd } root of the olfactory nerve.
_II_ Optic nerve.
_III_ Oculo-motor nerve.
_IV_ Trochlear nerve.
_VI_ Abducens nerve.
_V_, _VII_, _VIII_ Trigeminal, facial, and
auditory nerves.
_IX_, _X_, _XI_ Glossopharyngeal, pneumogastric,
and accessory nerves.
]

α. *External form* (Figs. 98, 102, 103 _L.op_). The optic lobes are two prominent ovoid bodies placed immediately in front of the _cerebellum_, and connected with it by the _valvula cerebelli_; posteriorly they touch each other in the median plane, while anteriorly they diverge and thus constitute the widest part of the brain; in the angle thus formed is the thalamencephalon. The optic lobes are always more darkly pigmented than any other part of the central nervous system.

Lateral view of brain of _Rana esculenta_.

_Ad_ Choroid plexus.
_C_ Cerebellum.
_Gp_ Pineal body.
_Hc_ Cerebral hemisphere.
_Hy_ Pituitary body.
_L.ol_ Olfactory lobe.
_L.ol_^1 Disc at origin of second root.
_L.op_ Optic lobe.
_Mo_ Medulla oblongata.
_Tho_ Thalamencephalon.
_To_ Optic tract.
_Tu.c_ Tuber cinereum.
_I_ 1st }
_I^1_ 2nd } root of olfactory nerve.
_II_ Optic nerve.
_IV_ Trochlear nerve.
_V_ Trigeminal nerve.
_VII_ Facial nerve.
_VIII_ Auditory nerve.
_IX_, _X_, _XI_ Glossopharyngeal, pneumogastric,
and accessory nerves.
]

The _Crura cerebri_ are two columns of white matter, placed beneath the optic lobes, and partly hidden by the pituitary gland. At their junction with the _medulla oblongata_, or rather with the _pars commissuralis_, is a very slight transverse fissure; at the same point the ventral longitudinal fissure is interrupted by an extremely small grey tubercle (Stieda).

β. *Internal structure.* From the anterior extremity of the fourth ventricle a canal, the Sylvian aqueduct (_Aqueductus Sylvii_, _iter a tertio ad quartum ventriculum_), may be traced forwards under the cerebellum, in the median line of this section of the brain. At about opposite the middle of the length of the optic lobes the canal is dilated and communicates with the cavities or ventricles (_Ventriculi lobi optici_, Stieda) enclosed by these; a general cavity is formed, which in transverse section has something of the form of the letter T. The roof of the cavity is thinner than the floor; this is especially the case in the median plane opposite the superior longitudinal fissure between the optic lobes; the floor is thinned in the middle line by the descending portion of the cavity. The cavity of each optic lobe extends both forwards and backwards beyond its point of communication with the dilated Sylvian aqueduct, hence in a transverse section taken in front of this point (Fig. 104 _h_′) the cavity of either side appears to be isolated; in a horizontal and longitudinal section (Fig. 105 _Aq_) the general arrangement of the parts may be well seen.

The grey matter is chiefly arranged in a layer so as to surround the cavity (Fig. 104), this layer being deeper on either side of the descending portion of the cavity than elsewhere. For the rest the grey matter is much interspersed among the white matter, except at the circumference of the section, which is entirely formed of white matter. It contains a large number of small cells, of which the nuclei are alone visible; in the parts mentioned where the layer is most marked these cells are arranged in oblique rows, between which pass fine bundles of medullated fibres (Fig. 104, between _u_ and _h_′). A group of large cells (Fig. 104 _u_) is found on either side of the middle line and under the floor of the cavity; the cells are about 0.032 mm. long, and 0.016 mm. broad; the oculo-motor nerve may be traced to this group, which is the *oculo-motor nucleus*. A small commissure of decussating fibres connects the nuclei of opposite sides (Köppen).

Transverse section through the anterior portion of the optic lobes opposite the origin of the motor-oculi nerve; from Stieda (magnified 30–80).

_h_ Lower portion of cavity.
_h_′ Lateral portion of cavity.
_u_ Ganglia of oculi-motor nerve.
_v_ Oculo-motor nerve.
_w_ Large cells of the optic lobe.
_x_ Roof of optic lobe.
_y_ Posterior commissure.
_z_ Anterior diverticulum of the cavity.
]

The substance of the roof of the optic lobes (Fig. 104 _x_) is arranged in very distinct layers: above is a layer free from cells; the fibres of this layer are extremely fine and delicate, and have not been accurately traced; under this is a nuclear layer; a second layer of fine fibres follows, which is succeeded by a second layer of nuclei in a granular matrix, and lastly epithelium (Stieda). (Reissner describes three nuclear layers in _Bufo variabilis_, and this is also the case in _R. temporaria_, G. H.; according to Köppen, the number is variable.) In the anterior portion of the roof a distinct bundle of fibres may be made out; externally they bend downwards, and can be traced as far as the _crura cerebri_.

Fibres corresponding with the commissural and arched fibres of the _medulla oblongata_ are continued into the hinder portion of the _crura cerebri_, the change from _pars commissuralis_ to _crura cerebri_ being very gradual. The longitudinal white fibres are much increased in number in the _crura cerebri_, and a portion of them can be traced to the _nucleus magnus_.

A peculiar irregular group of large cells (Fig. 104 _w_) is found where the roof meets the _crura cerebri_; these vary in diameter from 0.024 to O.040 mm., and their processes are very indistinct.

The fibres of the optic tracts arise, according to Köppen, from two different origins: the one lies on the hindermost part of the optic lobe; from this point the fibres curve downwards and forwards to form longitudinal fibres; this root Köppen names the ventral ascending root, it can be traced through the entire length of the organ. The second root arises in the _tectum opticum_ near the longitudinal fissure; it is smaller than the foregoing, and has been named the dorsal ascending optic root. The fibres of these two roots unite anteriorly near the posterior commissure, at which point they receive additional fibres (Köppen). Bellonci traces a large proportion of the fibres of the optic tract to the _nucleus magnus_, which pair of nuclei, as already stated, he regards as the posterior pair of _corpora quadrigemina_ of higher animals.

The _pars peduncularis_ is the continuation of the _pars commissuralis_ underneath the optic lobes; a gelatinous mass lying in the median plane and containing numerous isolated nuclei (_Ganglion interpedunculare_) divides it into two lateral halves. The longitudinal fibres are ungrouped posteriorly, but arranged in rounded strands in the middle, especially dorsally; anteriorly the grouped arrangement is lost and the number of fibres diminished.

The *pigment* of this region has, in a transverse section, an outline which has something the form of a lyre; commencing on either side of the median line, and underneath the deepest portion of the cavity, the pigment line passes, first, directly outwards; then suddenly turns upwards and slightly outwards parallel with the wall of the deeper part of the cavity; it then curves outwards to pass below the cavity of the optic lobe, where it divides, one portion passing outwards, the other between the Sylvian aqueduct and the ventricle of the optic lobe.

*d.* The *Thalamencephalon* (Huxley), (_Lobus ventriculi tertii_, Stieda; _Thalami optici_, Reissner; _Thalamus opticus s. Lobus ventriculi tertii_, Stannius; Ganglien der Haemisphaeren, Carus).

α. *External form.* From above (Fig. 98 _Tho_) the thalamencephalon is seen as a lozenge-shaped mass lying in front of the optic lobes, and behind and between the diverging posterior ends of the cerebral hemispheres; it is covered by a thick vascular membrane, the *choroid plexus*, through which passes the pedicle of the *pineal body* (_Glandula pinealis_). On removing the choroid plexus a small aperture is seen in the roof of the thalamencephalon, connecting the hollow pedicle of the pineal gland with the *third ventricle*. The ventricle appears as a narrow slit in the median line, its walls being formed by the optic thalami. By pressing aside the cerebral hemispheres the *posterior commissure* (_Commissura posterior_) may be seen lying quite in front and deep in the cleft of the ventricle. Immediately behind the pedicle of the pineal body is a slight but well-marked depression (Fig 98 _G_), the origin of which has not been investigated (Wiedersheim).

The choroid plexus is continued forwards between the cerebral hemispheres (Figs. 98, 103 _Ad_) for some distance, and terminates in a fine thread of connective-tissue.

The under surface of the thalamencephalon (Fig. 102) is divided into two parts by the *optic chiasma* (_Cho_): the anterior portion (_Lt_) is the _lamina terminalis_ (_Substantia cinerea anterior_); the posterior (_Tu.c_) the _tuber cinereum_. The _lamina terminalis_ is bounded on either side by the cerebral hemispheres. The _tuber cinereum_ (Figs. 102, 103 _Tu.c_) is a small median swelling immediately behind the optic chiasma, and caused by the depression of the floor of the third ventricle to form the _infundibulum_ (_Diverticulum infundibuli_, Reissner).

Horizontal section through the brain to show the ventricles.

_Aq_ Ventricles of optic lobes and the aqueduct of Sylvius.
_Dv_ Third ventricle.
_MF_ Foramen of Monroe.
_Sv_ Lateral ventricle.
_Vv_ Fourth ventricle.
]

The *pituitary body* (_Hypophysis cerebri_) is a flattened sac, placed behind the _tuber cinereum_ and continuous with it by means of the _infundibulum_.

β. *Internal structure.* The aqueduct of Sylvius, after communicating with the ventricles of the optic lobes, again contracts (Fig. 105), but still remains somewhat larger than before. In the thalamencephalon the Sylvian aqueduct opens into the third ventricle, which gradually assumes the form of a vertical slit with the walls bulging slightly outwards in their upper parts. The thin roof of this ventricle, where complete, contains a band of transverse fibres. The floor is depressed both before and behind the part immediately above the optic chiasma, the posterior depression lying above the _tuber cinereum_, which here descends towards the _infundibulum_: a transverse section through this portion of the third ventricle has the form of a square standing on one angle, the superior angle being produced into the vertical slit of the general cavity. The anterior depression is formed by the general cavity being prolonged downwards and forwards to the _lamina terminalis_ in the form of a narrow and shallow slit.

The white and grey matter of the thalamencephalon are only so far distinct in that the portion immediately surrounding the cavity is darker than the rest of the section. In the immediate neighbourhood of the cavity are many small cells and nuclei, which become scarcer further from the ventricle; they are arranged in rows, separated by a fibrillated matrix. On either side is a distinct bundle of longitudinal fibres, the ‘round bundle’ of Köppen, which come from the optic lobes but appear to arise from either the _pars commissuralis_ or the _medulla oblongata_, and to receive additional fibres from the optic thalami; they pass forwards to the base of the cerebral hemispheres (Stieda). A second set of longitudinal fibres arises in the substance of the _tuber cinereum_ and passes forwards to the hemispheres (strand of the _Tuber-cinereum_ and _Thalamus_, Köppen); this band, together with the ‘round bundle,’ form a _crus cerebri_ (Köppen).

The commissural fibres of the thalamencephalon are: (1) a _commissura transversa Halleri_ in the posterior portion of the organ; (2) an optic commissure, consisting of fibres arising from the thalamencephalon (thalamencephalic root) and crossing the median line to join the optic tract of the opposite side; (3) a probable commissure between the optic nerves just in front of the _chiasma opticorum_; the existence of the latter is not yet proved beyond doubt; (4) the large commissure of transverse fibres found in the roof of the third ventricle; whether the fibres decussate or not is uncertain (Köppen).

The fibres of the large commissure of the roof are, in part, continued into the strand of the _Tuber-cinereum_ above mentioned, and thus conducted to the posterior parts of the hemispheres (Köppen). A group of fibres (Meynert’s band, Köppen) is found in each lateral wall of the third ventricle; they pass from the region of the _nucleus parvus_ downwards in a curved course almost parallel with the external border of the thalamencephalon.

A distinct group of cells may be noted in this section of the brain, an arciform or circular group (_Nucleus parvus_, Reissner; ganglion of the _habenula_, Köppen) of large spindle cells (average diameter 0.016 mm.), placed under the upper border close to the third ventricle; the group extends alongside the whole length of the ventricle. A second group lying in the middle and posterior parts beneath the ventricle, Köppen names the ‘ventral nucleus’ of the thalamencephalon.

The *pigment* in the posterior part of the thalamencephalon is arranged in a manner similar to that in the optic lobes and _crura cerebri_; anteriorly where the third ventricle is prolonged forwards and downwards the arrangement is different; the pigment lies in a curved line above the process of the ventricle, with its concavity directed downwards, each end bifurcating, in order that one branch may pass upwards, the other downwards.

The *pineal body*[53] is a small vesicle placed underneath the skin above the fronto-parietal bones; in the embryo it is connected with the third ventricle by means of the pedicle[54] already mentioned; the skin covering the body is always paler than the surrounding skin, and the usual cutaneous glands are absent in this part; the paler spot on the head may always be found, but is more distinct in _Rana temporaria_ than in _Rana esculenta_. The structure on the roof of the third ventricle, which is usually known as the pineal body, is nothing more than a thickened portion of the choroid plexus, and consists of a group of convoluted vessels surrounded by _pia mater_, which is described by Wyman as being covered with ciliated epithelium (_R. pipiens_). The true pineal body is a small body with an outer connective-tissue capsule, derived from the _pia mater_; this encloses an irregular mass of epithelial cells; according to de Graaf a twig of the _ramus supramaxillaris_ reaches it subcutaneously, and a blood-vessel accompanies the pedicle through the _foramen parietale_. According to Darkschewitsch, the pedicle contains medullated nerve-fibres derived directly from the brain.

[Footnote 53: Compare Ehlers, Ueber die Zirbel der Haifische, Zeit. f. wiss. Zool. 1878, Vol. XXX; and Balfour, Development of the Elasmobranch Fishes, chap. ix.]

[Footnote 54: Wiedersheim states that the pedicle is hollow, and regards the part formerly known as the pineal body as a thickened portion of this pedicle.]

The *pituitary body* (Figs. 102, 103 _Hy_) when examined with a lens is seen to consist of two portions: an anterior, superior, and smaller white portion, and a larger, inferior, posterior, and reddish portion. The anterior portion has the form of a very small, flat disk, and is enclosed in a connective-tissue capsule which sends in larger and smaller processes. In either transverse or longitudinal section it is seen to be formed of two horizontal layers separated by a line of blood-vessels and connective-tissue. The upper layer consists of a granular and reticular matrix, containing many nuclei (averaging 0.006 to 0.010 mm. diameter), and divided into irregular rounded or polyhedral spaces by bands of tissue derived from the capsule. This layer is more vascular than the lower. The lower layer consists of a mass of clear, nucleated rounded or polyhedral cells (0.016 to 0.024 mm. in diameter; nuclei from 0.008 to O.012 mm. in diameter, Reissner), pierced by very fine connective-tissue septa derived from the capsule. The septa are, for the most part, vertical and longitudinal (Reissner), the blood-vessels are very few.

The posterior larger portion of the pituitary body (Fig. 106) is also compressed from above downwards, and in transverse section as an oval outline. It possesses an external thin connective-tissue capsule, which sends in fine processes to support a mass of convoluted tubes, between which course a few blood-vessels; these tubes possess an outer nucleated basement-membrane, and are lined with a single layer of more or less cylindrical epithelium, which entirely fills the tube; hence the tubes possess no lumen. The tubes are from 0.04 to 0.08 mm. in diameter; the cells are clear or granular, and possess distinct, rounded nuclei.

*e.* The *Cerebral Hemispheres* and *Olfactory Lobes*. The cerebral hemispheres (_Lobi hemisphaerici_, Stieda; _Lobi cerebrales_, Reissner; Centralmasse des Geruchssinns, Carus; Hemisphaeren des grossen Hirns, Tiedemann; Grosse Hemisphaeren, Schiess; Prosencephalon, Huxley). The olfactory lobes (_Tubercula olfactoria_, Stieda; _Lobi olfactorii_, Reissner; Riechkolben, Schiess; Rhinencephalon, Huxley).

α. *External form* (Figs. 98, 102, 103 _Hc_ and _L.ol_). The two cerebral hemispheres form together the largest section of the brain; from above they are seen to be separated by a dorsal longitudinal fissure, which is here well marked: each hemisphere is an ovoid body with the smaller end directed forwards and continuous with the corresponding olfactory lobe; the posterior end forms one half of the anterior boundary of the thalamencephalon. The olfactory lobes are two elongated, rounded bodies directly continuous with the corresponding cerebral hemispheres, and likewise partially separated in the median line by a dorsal longitudinal fissure: at the point of union of the cerebral hemispheres and olfactory lobes is a faintly marked transverse depression.

Section through the lower division of the pituitary body (magnified 360 times); from Stieda.

_a_ Tubes lined with epithelium.
_b_ Blood-vessels.
]

On the ventral surface the parts are again marked off from one another by a corresponding ventral longitudinal fissure and a transverse groove; the two longitudinal fissures being continuous, anteriorly, between the olfactory lobes. The cerebral hemispheres appear to be more widely separated behind (Fig. 102) than is the case on the dorsal surface, and in the space so formed is the _lamina terminalis_ (_Lt_). Seen from the side, the slight depression of the upper surface, between the cerebral hemispheres and the olfactory lobes, is seen to be continued downwards and slightly backwards to join the corresponding groove on the inferior surface.

The longitudinal fissures are shallow and do not meet, except at one point, at about the middle of the cerebral hemispheres (Fig. 105). The olfactory bulbs arise superficially (Figs. 102, 103 _I_′ and _L.ol_) from the whole length of the olfactory lobe, between the anterior extremity (_I_′) and the posterior (_L.ol_), where they are also attached to the cerebral hemispheres.

β. *Internal structure.* The cerebral hemispheres and olfactory lobes are hollow, the common cavity of each side being known as the lateral ventricle (_Ventriculus lateralis_); these ventricles communicate with each other, and with the third ventricle (Fig. 105). The narrow aperture by which the lateral ventricles communicate is known as the *Foramen of Monro* _(MF_); it communicates with the third ventricle posteriorly, and with the space between the cerebral hemispheres anteriorly, and thus forms a common cavity (_Ventriculus communis loborum hemisphaericorum_, Stieda).

In general terms, each ventricle may be said to be a semilunar cavity, prolonged backwards and forwards (Fig. 107 _c_); the outer wall is always more or less concave, while the inner varies according to the part of the hemisphere examined. At the anterior and posterior extremities the inner wall is convex and bulges into the cavity (Fig. 107); in the middle portion of the cavity the inner wall presents a longitudinal groove (_Ventriculi lateralis cornu internum_, Reissner), (Fig. 108 _d_), and consequently the inner wall has here two rounded masses, an upper and a lower (_Corpus striatum_, Wiedersheim), projecting into it. By tracing them backwards and forwards, the lower swelling is seen to increase at the expense of the upper, while at the same time the lateral grooves disappear; the outline of the cavity shown in Fig. 107 is then obtained. The roof of the ventricle is arched and broader than the floor, which, in the middle part, exists only as a vertical slit (Fig. 108), (_Ventriculi lateralis cornu inferius_, Reissner): towards the anterior and posterior extremities it widens and becomes shallower (Fig. 107).

Transverse section through the hinder portion of the cerebral hemispheres; from Stieda.

_c_ Lateral ventricle.
_c_′ Common ventricle of Stieda.
_d_ Longitudinal fibres.
_f_ Anterior prolongation of the third ventricle.
]

The cerebral hemispheres and olfactory lobes are composed of a fine granular matrix, enclosing spindle-shaped, rounded, or pear-shaped nerve cells and nuclei, and containing very fine fibres. The cells (Fig. 109 _b_) are more numerous towards the ventricle, and somewhat sparse towards the superficial surface. The cells are of two chief sizes, the smaller and more numerous average 0.004 mm. to 0.008 mm. in diameter; they are found chiefly in the deeper portions of the section, but also form a very thin irregular layer beneath the _pia mater_: the larger cells have an average diameter of 0.010 mm. to 0.012 mm., and are placed towards the periphery, especially in the dorsal part of the inner wall.

Transverse section near the middle of the cerebral hemispheres.

_c_ Lateral ventricle.
_d_ Groove on the inner wall.
]

In this irregular collection of cells the following centres have been described: (1) The nucleus, through which the _corpus callosum_ passes (Köppen); (2) the lower internal or median cell-area (Osborne), situated above the foregoing nucleus in the posterior and middle portions of the hemispheres; (3) the upper internal cell-area (Osborne) is the area of large cells in the dorsal part of the inner wall; (4) the _Corpus striatum_ (Osborne) is a mass of cells between the _corpus callosum_ and the _commissura anterior_; Köppen doubts the correctness of Osborne’s opinion, and suggests that a group of cells found in the wall of the third ventricle in front of ‘Meynert’s band’ may perhaps be a _corpus striatum_.

From a transverse section through one of the cerebral hemispheres; from Stieda. (Magnified 360 times.)

_a_ Epithelium of lateral ventricle.
_b_ Nerve cells.
_c_ Connective-tissue processes from the Pia mater.
]

A bundle of longitudinal medullated nerve-fibres, the ‘round bundle,’ is found on either side of the median line (Fig. 107 _d_), and near the lower border in the posterior portion of this region; these fibres can be traced from the posterior section of the thalamencephalon; they diminish in number as they course through the base of the cerebral hemispheres, and are ultimately lost in the lower anterior part of the outer walls. A second strand of longitudinal fibres is the continuation of that of the _Tuber-cinereum_ above described, which can be followed to the outer wall of the posterior part of the ventricles, and to the anterior commissure.

The commissures are: (1) The _corpus callosum_, a large bundle of transverse fibres, seen best in a transverse section, at the point of junction between the _lamina terminalis_ and the cerebral hemispheres, forming an arch over the roof of the anterior prolongation of the third ventricle. The fibres course to the inner and anterior parts of the hemispheres, and are situated chiefly behind the _foramen Monroi_. To this commissure must be added those fibres which unite the two olfactory lobes, and possibly the fibres (_Commissura posterior_) found in the roof of the third ventricle (Köppen). (2) The _Commissura anterior_ (Stieda), a smaller set, found immediately under the floor of the common ventricle, forming in their course outwards a curve, with the concavity directed downwards. This commissure connects the ‘round bundles’ of opposite sides, and those fibres coursing with the ‘round bundles’ to the olfactory lobes constitute the _pars olfactoria_ of the _commissura anterior_. To this commissure must also be added some fibres found on the ventral surface of the _commissura anterior_ and connecting the two strands of the _Tuber-cinereum_; an unusually coarse strand of these fibres can be traced to the inner wall of the ventricle, and is termed the _pars olfactoria interna_ by Osborne.

The general structure of the olfactory lobes resembles that of the hemispheres; the olfactory nerves arise each by two roots, an outer and inner. The outer root arises from the outer wall near the groove between the corresponding hemisphere and olfactory lobe; the inner or anterior root arises from the anterior surface of the olfactory lobe. Both roots have a peculiar method of origin from the extremely fine fibrillar network of the matrix (Nerve-fibre-conglomerate, Köppen), in which are rounded dark bodies known as ‘glomeruli;’ in the ‘glomeruli’ dark points and nuclei are seen, between larger and smaller bands of nerve-fibres. Köppen holds that all the sensory nerves of the brain arise in a similar manner.

A decussation takes place between the two inner roots of the olfactory nerves; possibly the external roots are connected by means of the _commissura anterior_.

Very little *pigment* exists in the cerebral hemispheres or olfactory lobes, the greater portion is found in the upper part of the inner walls of the cerebral hemispheres.

The *epithelium* of the *ventricles of the brain*, like that of the central canal of the *spinal cord*, consists of conical cells with their bases directed towards the cavity, and their apices directed peripherally and prolonged into distinct processes (Figs. 104, 109 _a_). In such situations, as the choroid plexuses, where nervous tissue is absent and the cavity is completed by _pia mater_ alone, the epithelial cells are flattened. Everywhere else it is ciliated[55] and possesses distinct round nuclei which are as broad as the cells themselves. The epithelium is somewhat irregularly and sparsely pigmented; the ventral parts of the central canal of the spinal cord, of the fourth ventricle, and of the Sylvian aqueduct are always more pigmented than the dorsal parts.

[Footnote 55: Schmidt (_l. c._) states that the epithelium of the central canal of the spinal cord is not ciliated.]

PLAN OF THE ARRANGEMENT OF THE NERVE-FIBRES OF THE CENTRAL NERVOUS SYSTEM, ACCORDING TO KÖPPEN.

NERVE FIBRE NERVE-FIBRE COMMISSURES. NERVE-FIBRES
STRANDS IN STRANDS IN WHICH CONNECT
THE WHITE THE GREY THE STRANDS OF
MATTER. MATTER. WHITE FIBRES.

{1. The ventral 1. Longitudinal 1. The ventral
{ columns, fibres in commissure.
{ including the dorsal 2. The dorsal
{ the strand portion of commissure
{ of coarse the grey (rudimentary).
The { fibres. matter.
spinal {2. The lateral
cord. { columns.
{3. The dorsal,
{ columns
{ including
{ Goll’s
{ column.

{1. The ventral 1. Longitudinal 1. The ventral 1. The fibrae
{ columns, fibres of the commissure: arcuatae.
{ including Substantia _a._ Dorsal
{ the strand gelatinosa portion.
{ of coarse Rolandi, and _b._ Ventral
{ fibres which give portion.
{ dorsal off ascending 2. (The dorsal
{ longitudinal vagal and commissure
{ strand. trigeminal is wanting.)
{2. The lateral roots.
{ columns,
The { which give
medulla { off an as-
oblongata.{ cending
{ (recurrent)
{ vagus-root.
{3. The dorsal
{ columns.
{ _a._ Ascending
{ trigeminal
{ root.
{ _b._ Auditory
{ root.
{ _c._ Descending
{ auditory
{ root.

{1. The ventral 1. Longitudinal 1. The ventral 1. The fibrae
{ columns, fibres of the commissure. arcuatae.
{ including grey matter. 2. The dorsal
{ the dorsal Small ascend- commissure,
{ longitudinal ing trigeminal cerebellar
{ strand (the root. commissure
{ strand of and trochlear
{ coarse fibres decussation.
The { has dis-
cere- { appeared).
bellum {2. The lateral
and the { columns (each
pars { receives a
commis- { strand from
suralis. { the cere-
{ bellum).
{3. The dorsal
{ columns.
{ _a._ Large as-
{ cending tri-
{ geminal root.
{ _b._ Descending
{ auditory
{ root.

{1. The ventral (Wanting.) 1. The ventral 1. The fibrae
{ columns, commissure. arcuatae.
{ including the 2. The dorsal
{ dorsal longi- commissure.
{ tudinal _a._ Commissura
{ strand. posterior.
{2. (The lateral _b._ Marginal
The { columns can- commissure.
optic { not now be
lobes. { distinguished
{ from the ven-
{ tral columns.)
{3. The dorsal
{ columns: two
{ ascending
{ opticus-roots.

{1. The ventral (Wanting.) 1. The ventral 1. The fibrae
{ and dorsal commissure: arcuatae.
The { columns. _a._ Commissura
thalam- { _a._ Fibres from transversa
enceph- { the optic Halleri.
alon and { lobes. _b._ Commissura
the cere- { _b._ The ‘round anterior.
bral hemi-{ bundle.’ 2. The dorsal
spheres. { _c._ The ‘Thal- commissure,
{ amus-Tuber- the corpus
{ cinereum callosum.
{ strand.’

C. THE COVERINGS OF THE BRAIN AND SPINAL CORD.

1. The _Pia mater_ closely invests the whole of the brain and spinal cord; it may, in fact, be regarded as the flattened, outermost layer of the connective-tissue material which supports the nervous elements; fine processes (Fig. 109 _c_) pass radially from its inner surface to join the general connective-tissue matrix or neuroglia. It is usually pigmented, those portions covering the optic lobes and spinal cord being especially deeply pigmented: on the cerebral hemispheres it contains very little pigment; at times, indeed, in _R. temporaria_, pigment is absent from this part.

The _pia mater_ is continued on to the choroid plexuses and pituitary body, and on to the pineal body by means of the pedicle; the membrane is very vascular, and forms, especially for the cerebral hemispheres and the olfactory lobes, a very important source for the direct supply of blood-vessels.

2. The _Dura mater_, or lining membrane of the cranial cavity and vertebral canal, is a connective-tissue membrane containing many very much branched, pigmented cells. This membrane is not so deeply pigmented as the _pia mater_, except in that portion covering the cerebral hemispheres and the olfactory lobes, which is much darker than the corresponding portion of the _pia mater_.

3. The *arachnoid membrane* is the layer of endothelial cells covering the inner surface of the _dura mater_: by means of the blood-vessels and nerves, etc., it is continued on to the _pia mater_ of the spinal cord and brain, which it in like manner closely invests. Masses of *calcareous crystals* are found between the epineurium and the arachnoid (Wiedersheim) on each spinal nerve at its exit from the intervertebral foramen, also on the trigeminal nerves in the cranium. Additional smaller and more irregular masses are found on the dorsal part of the _dura mater_ of the spinal cord.

D. THE VESSELS OF THE BRAIN AND SPINAL CORD. (Fig. 110.)

The arteries of these organs are derived from the internal carotids and the _arteriae vertebrales_. As soon as the internal carotid arteries reach the cranial cavity each divides into two branches, an anterior (_Ramus anterior_, Schöbl[56]) and a posterior (_Ramus posterior_, Schöbl); the anterior branches course forwards on the lower part of the surface of the brain as far as the anterior portion of the thalamencephalon, where they again divide; one division, _arteria lobi hemisphaerici inferior externa_ (Schöbl), continues forwards along the outer surface of the cerebral hemisphere and of the olfactory lobe as far as the olfactory bulb, where it may still be distinctly seen; it gives off branches to the adjacent parts in its course forwards. The vessels of the two sides communicate with each other by means of delicate transverse vessels (_Arteriae communicantes anteriores_). The second division, _arteria lobi hemisphaerici superior interna_ (Schöbl), of the _ramus anterior_ courses on the thalamencephalon to the dorsal surface, gives a large branch to the choroid plexus of the fourth ventricle, and runs forwards in the dorsal longitudinal fissure; it supplies vessels to all the neighbouring parts.

[Footnote 56: These names are those adopted by Schöbl for corresponding vessels in the newt, the translator has accepted them as being suitable, with slight modification, to the vessels of the frog.]

The _rami posteriores_ converge as they course backwards, and ultimately unite to form an _arteria basilaris_, which is continued in the median line of the under surface of the spinal cord as the _arteria spinalis anterior_. The _ramus posterior_ gives off, in its course, many small vessels to the neighbouring parts, and two larger vessels on either side, one of which, the _arteria lobi optici_ (Schöbl), is distributed to the optic lobes, while the other passes to the pituitary body.

The branches of these vessels form a network in the _pia mater_, from which the nervous tissue is supplied; they also send numerous branches directly into the brain and cord, and these have a similar arrangement for all parts of the brain with the exception of the cerebral hemispheres and olfactory lobes; more or less vertical branches arise from the posterior parts of the upper borders of the _rami anteriores_, from the whole of the _rami posteriores_, and from the _arteria basilaris_; these course upwards from their place of origin into the corresponding part of the brain, give off a few branches in the white matter, and then branch freely, and at somewhat sharp angles, in the grey matter. In the _pars commissuralis_ a large branch may be traced from the _ramus posterior_ on either side upwards into the cerebellum almost to its upper border. The vessels of the cerebral hemispheres and olfactory lobes seem to possess no other definite arrangement than that described above. The grey matter seems, on the whole, to be more vascular than the white; the vessels of the latter are chiefly arranged radially to the surface, and run in courses which are more or less straight; the vessels of the grey matter are more irregular and sinuous.

The _Arteria spinalis anterior_ courses along the whole length of the spinal cord, giving off lateral branches and communicating with _rami spinales_ (branches of the _Arteria vertebralis_). These branches form a plexus in the _pia mater_, from which vessels pass at irregular points into the cord; one set of small vessels, described by Reissner, pass in a straight course from the superior longitudinal sinus towards the _substantia reticularis_, where they divide. Other branches pass directly from the _arteria spinalis anterior_, through the ventral longitudinal fissure, and there divide; the twigs as a rule avoiding the _septum medium_ and passing in greater part towards the ventral horns. The vessels in the white matter are for the most part radial and straight, while in the more vascular grey matter they are irregular and more sinuous in their course.

Diagram to show the _Vena spinalis posterior_, the _Venae spinales superiores_, and the origin of the _Vena jugularis interna_.

_H_ Hemispheres.
_ji_ Internal jugular.
_op_ Optic lobes.
_mo_ Medulla oblongata.
_spp_ Vena spinalis posterior.
]

The choroid plexus of the third ventricle (_Plexus choroideus ventriculi tertii_, Reissner) lies, as already described, on the roof of the third ventricle; it is somewhat triangular in form (_Plexus venosus triangularis_, Schöbl), and is evidently a prolongation of the _pia mater_, with an increased supply of vessels. It receives, at its anterior angle, veins from the cerebral hemispheres and the adjacent parts. The under surface is covered with a layer of ciliated pavement epithelium. At the posterior angles of the plexus the veins communicate on either side (Fig. 110) with the anterior lateral prolongation of the _vena spinalis superior_, and with the internal jugular vein. The small body usually named the pineal body, and placed on the roof of the ventricle, is only a small thickened portion of the plexus, and consists of a group of convoluted blood-vessels. This plexus sends a considerable bunch of vessels into the upper part of the third ventricle. The choroid plexus of the fourth ventricle (_Plexus choroideus ventriculi quarti_, Reissner) is a triangular membrane, slightly attached to the borders of the fourth ventricle. Its upper surface is flat and bounded at either side by a large vein, the _vena spinalis superior_; anteriorly it is bounded by the _cerebellum_. The under surface is not flat; in the middle line is a slight furrow corresponding to the position of a median vessel, which may, when injected, be seen from the dorsal surface; from it a number of short vessels pass outwards and slightly backwards to the outer border. The connective-tissue between these vessels is pushed down into the cavity; in this manner one obtains a double row of flattened villous-looking bodies, which frequently have their tips pigmented. The under surface of the plexus is clothed with flattened, ciliated epithelium.

On either side of the pituitary body is another small venous plexus (_Plexus lateralis_, Schöbl); it communicates above with the posterior angle of the choroid plexus of the third ventricle; externally, with the internal jugular vein; and internally with its fellow of the opposite side by more or less irregular transverse vessels, which, together with a median vein from the fissure between the cerebral hemispheres, form an irregular _circulus venosus_ around the pituitary body. These plexuses receive blood from the lower part of the cerebral hemispheres, the thalamencephalon, the optic lobes, and from the pituitary body. The veins of the spinal cord open into the dorsal, _vena spinalis posterior_ (Fig. 110 _spp_), which bifurcates at the posterior angle of the fourth ventricle, one division passing to either side and forming, as already described, the outer border of the choroid plexus; anteriorly it opens into the internal jugular vein (Fig. 110 _ji_).

The blood-vessels, both arteries and veins, are irregularly pigmented, both on the surface and in the interior of the central nervous system; those of the cerebral hemispheres and olfactory lobes have very little or no pigment.

DESCRIPTION OF THE FIGURES ON PLATE I.

Fig. 111. Dorsal view of the orbit, etc.; deep dissection.

_c_ Course of the palatine nerve.
_db_ Fibrous plate.
_dm_ M. depressor maxillae.
_ef_ Terminal branches of the ophthalmic nerve.
_F_ Facial with R. anterior of the glossopharyngeal.
_g,g,g_ Terminal twigs of the ophthalmic nerve.
_h_ Muscular twigs of the Ram. maxillaris.
_ics_ M. intertransversar. capitis superior.
_ii_ Twigs of upper eyelid.
_K_ Terminal twigs of the Ram. maxillaris.
_la_ M. levator anguli scapulae.
_pt_ M. pterygoideus.
_sc_ M. sterno-cleido-mastoideus.
_Sp_ Sympathetic nerve.
_t_ M. temporalis.
_tr_ Nasal branch.

Union of facial nerve with Ram. anterior of the glossopharyngeal
nerve.

_I_ Olfactory nerve.
_II_ Optic nerve.
_III_ Motor oculi nerve.
_IV_ Trochlear nerve.
_Vb_ R. palatinus }
_Vc_ R. maxillo-mandibularis } of the trigeminal
_Vc_′ R. maxillaris } nerve.
_Vd_ R. mandibularis }
_Ve(VII)_ Facial nerve.
_Vg_ Gasserian ganglion.
_VN_ Trigeminus.
_VS_ Sympathetic nerve.
_VI_ Abducens nerve.
_VII_′ Facial nerve.
_VIII_ Auditory nerve.
_X1_ Ram. anterior of the glossopharyngeal.
_X2_ Glossopharyngeal nerve.
_X3_ Pneumogastric nerve.
_XG_ Ganglion nervi vagi.
_XI_ Accessory nerve.

Fig. 112. Dorsal view of the orbit, etc.; superficial dissection.

_e_ External branch of the ophthalmic nerve.
_f_ Internal branch of the ophthalmic nerve.
_g,g,g_ Terminal twigs of the ophthalmic nerve.
_II_ Optic nerve.
_III_ Motor oculi nerve.
_IV_ Trochlear nerve.
_Va_ Ophthalmic nerve.
_Vc_ R. maxillo-mandibularis.
_Vc_′ R. maxillaris of the trigeminal nerve.
_Vd_ R. mandibularis of the trigeminal nerve.
_VI_ Abducens nerve.
_VI_′ Ciliary nerves.

Fig. 113. View of roof of mouth; mucous membrane, etc., removed.