Chapter VI: Neurology

The most simple condition of the nervous central organ known in Vertebrates is found in _Branchiostoma_. In this fish the spinal chord tapers at both ends, an anterior cerebral swelling, or anything approaching a brain, being absent. It is band-like along its middle third, and groups of darker cells mark the origins of the fifty or sixty pairs of nerves which accompany the intermuscular septa, and divide into a dorsal and ventral branch, as in other fishes. The two anterior pairs pass to the membranous parts above the mouth, and supply with nerve filaments a ciliated depression near the extremity of the fish, which is considered to be an olfactory organ, and two pigment spots, the rudiments of eyes. An auditory organ is absent.

The _spinal chord_ of the _Cyclostomes_ is flattened in its whole extent, band-like, and elastic; also in _Chimæra_ it is elastic, but flattened in its posterior portion only. In all other fishes it is cylindrical, non-ductile, and generally extending along the whole length of the spinal canal. The Plectognaths offer a singular exception in this respect that the spinal chord is much shortened, the posterior portion of the canal being occupied by a long cauda equina; this shortening of the spinal chord has become extreme in the Sun-fish (_Orthagoriscus_), in which it has shrunk into a short and conical appendage of the brain. Also in the Devil-fish (_Lophius_) a long cauda equina partly conceals the chord which terminates on the level of about the twelfth vertebra.

The _brain_ of fishes is relatively small; in the Burbot (_Lota_) it has been estimated to be 1/720th part of the weight of the entire fish, in the Pike the 1/1305th part, and in the large Sharks it is relatively still smaller. It never fills the entire cavity of the cranium; between the dura mater which adheres to the inner surface of the cranial cavity, and the arachnoidea which envelops the brain, a more or less considerable space remains, which is filled with a soft gelatinous mass generally containing a large quantity of fat. It has been observed that this space is much less in young specimens than in adult, which proves that the brain of fishes does not grow in the same proportion as the rest of the body; and, indeed, its size is nearly the same in individuals of which one is double the bulk of the other.

I. Upper aspect. II. Lower aspect.

_a_, cerebellum; _b_, optic lobes; _c_, hemispheres; _e_,
lobi inferiores; _f_, hypophysis; _g_, lobi posteriores; _i_,
Olfactory lobes; _n_, _N._ opticus; _o_, _N._ olfactorius;
_p_, _N._ oculo-motorius; _q_, _N._ trochlearis; _r_, _N._
trigeminus; _s_, _N._ acusticus; _t_, _N._ vagus; _u_, _N._
abducens; _v_, Fourth ventricle.]

The brain of _Osseous fishes_ (Fig. 41) viewed from above shows three protuberances, respectively termed _prosencephalon_, _mesencephalon_, and _metencephalon_, the two anterior of which are paired, the hindmost being single. The foremost pair are the _hemispheres_, which are solid in their interior, and provided with two swellings in front, the _olfactory lobes_. The second pair are the _optic lobes_, which generally are larger than the hemispheres, and succeeded by the third single portion, _the cerebellum_. In the fresh state the hemispheres are of a grayish colour, and often show some shallow depressions on their surface; a narrow commissure of white colour connects them with each other. The optic lobes possess a cavity (_ventriculus lobioptici_), at the bottom of which some protuberances of variable development represent the corpora quadrigemina of higher animals. On the lower surface of the base of the optic lobes, behind the _crura cerebri_, two swellings are observed, the _lobi inferiores_, which slightly diverge in front for the passage of the _infundibulum_, from which a generally large _hypophysis_ or _pituitary gland_ is suspended. The relative size of the cerebellum varies greatly in the different osseous fishes: in the Tunny and Silurus it is so large as nearly to cover the optic lobes; sometimes distinct transverse grooves and a median longitudinal groove are visible. The cerebellum possesses in its interior a cavity which communicates with the anterior part of the fourth ventricle. The _medulla oblongata_ is broader than the spinal chord, and contains the _fourth ventricle_, which forms the continuation of the central canal of the spinal chord. In most fishes a perfect roof is formed over the fourth ventricle by two longitudinal pads, which meet each other in the median line (_lobi posteriores_), and but rarely it remains open along its upper surface.

The brain of _Ganoid fishes_ shows great similarity to that of the Teleostei; however, there is considerable diversity of the arrangement of its various portions in the different types. In the Sturgeons and _Polypterus_ (Fig. 42) the hemispheres are more or less remote from the mesencephalon, so that in an upper view the crura cerebri, with the intermediate entrance into the third ventricle (_fissura cerebri magna_), may be seen. A vascular membranous sac, containing lymphatic fluid (_epiphysis_), takes its origin from the third ventricle, its base being expanded over the anterior interspace of the optic lobes, and the apex being fixed to the cartilaginous roof of the cranium. This structure is not peculiar to the Ganoids, but found in various stages of development in Teleosteans, marking, when present, the boundary between prosencephalon and mesencephalon. The lobi optici are essentially as in Teleosteans. The cerebellum penetrates into the ventriculus lobi optici, and extends thence into the open sinus rhomboidalis. At its upper surface it is crossed by a commissure formed by the _corpora restiformia_ of the medulla.

I., Upper; II., Lateral; III., Lower aspect.

_a_, Medulla; _b_, corpora restiformia; _c_, cerebellum;
_d_, lobi optici; _e_, hypophysis; _f_, fissura cerebri
magna; _g_, nervus opticus; _g_’, chiasma; _h_, hemispheres;
_i_, lobus olfactorius; _k_, sinus rhomboidalis (fourth
ventricle).]

As regards external configuration, the brain of _Lepidosteus_ and _Amia_ approach still more the Teleosteous type. The prosencephalon, mesencephalon, and metencephalon are contiguous, and the cerebellum lacks the prominent transverse commissure at its upper surface. The sinus rhomboidalis is open.

The brain of the _Dipnoi_ shows characters reminding us of that of the Ganoids as well as the Chondropterygians, _Ceratodus_ agreeing with _Protopterus_ in this respect, as in most other points of its organisation. The hemispheres form the largest part of the brain; they are coalescent, as in Sharks, but possess two lateral ventricles, the separation being externally indicated by a shallow median groove on the upper surface. The olfactory lobes take their origin from the upper anterior end of the hemispheres. Epiphysis and hypophysis well developed. The lobi optici are very small, and remote from the prosencephalon, their division into the lateral halves being indicated by a median groove only. The cerebellum is very small, overlying the front part of the sinus rhomboidalis.

_ac_, Nerv. acusticus; _b_, corpus restiforme; _c_,
cerebellum; _d_, lobus opticus; _e_, hypophysis; _g_, nervus
opticus; _h_, hemisphere; _i_, lobus olfactorius; _i’_,
olfactory pedicle; _k_, nerv. olfactorius; _l_, epiphysis;
_m_, nerv. oculo-motorius; _tr_, nerv. trigeminus; _v_, nerv.
vagus.]

The brain of _Chondropterygians_ (Fig. 43) is more developed than that of all other fishes, and distinguished by well-marked characters. These are, first, the prolongation of the olfactory lobes into more or less long pedicles, which dilate into great ganglionic masses, where they come into contact with the olfactory sacs; secondly, the space which generally intervenes between prosencephalon and mesencephalon, as in some Ganoids; thirdly, the large development of the metencephalon.

The hemispheres are generally large, coalescent, but with a median, longitudinal, dividing groove. Frequently their surface shows traces of gyrations, and when they are provided with lateral ventricles, tubercles representing _corpora striata_ may be observed. The olfactory pedicles take their origin from the side of the hemispheres, and are frequently hollow, and if so, their cavity communicates with the ventricle of the hemisphere. The optic lobes are generally smaller than the hemispheres, coalescent, and provided with an upper median groove like the prosencephalon. At their base a pair of lobi inferiores are constant, with the hypophysis and _sacsus vasculosus_ (a conglomeration of vascular loops without medullary substance) between them.

The cerebellum is very large, overlying a portion of the optic lobes and of the sinus rhomboidalis, and is frequently transversely grooved. The side-walls of the fourth ventricle, which are formed by the corpora restiformia, are singularly folded, and appear as two pads, one on each side of the cerebellum (_lobi posteriores_ s. _lobi nervi trigemini_).

I., Upper; II., Lower aspect. Letters as in Fig. 45.]

I., Upper; II., Lower aspect.

_a_, Medulla oblongata; _ac_, nerv. acusticus; _b_, corpus
restiforme or rudimentary cerebellum; _d_, lobus ventriculi
tertii; _d’_, entrance into the third ventricle; _c_,
hypophysis; _fa_, nerv. facialis; _g_, nerv. opticus;
_h_, hemisphere; _hy_, nerv. hypoglossus (so named by
Müller); _i_, lobus olfactorius; _k_, sinus rhomboidalis;
_l_, epiphysis; _m_, nerv. oculo-motorius; _q_, corpora
quadrigemina; _tr_, nerv. trigeminus; _tro_, nerv.
trochlearis; _v_, nerv. vagus.]

The brain of the _Cyclostomes_ (Figs. 44, 45) represents a type different from that of other fishes, showing at its upper surface three pairs of protuberances in front of the cerebellum; they are all solid. Their homologies are not yet satisfactorily determined, parts of the Myxinoid brain having received by the same observers determinations very different from those given to the corresponding parts of the brain of the Lampreys. The foremost pair are the large olfactory tubercles, which are exceedingly large in _Petromyzon_. They are followed by the hemispheres, with a single body wedged in between their posterior half; in _Petromyzon_, at least, the vascular tissue leading to an epiphysis seems to be connected with this body. Then follows the lobus ventriculi tertii, distinctly paired in Myxinoids, less so in _Petromyzon_. The last pair are the _corpora quadrigemina_. According to this interpretation the cerebellum would be absent in Myxinoids, and represented in _Petromyzon_ by a narrow commissure only (Fig. 45, _b_), stretching over the foremost part of the sinus rhomboidalis. In the Myxinoids the _medulla oblongata_ ends in two divergent swellings, free and obtuse at their extremity, from which most of the cerebral nerves take their origin.

The _Nerves_ which supply the organs of the head are either merely continuations or diverticula of the brain-substance, or proper nerves taking their origin from the brain, or receiving their constituent parts from the foremost part of the spinal chord. The number of these spino-cerebral nerves is always less than in the higher vertebrates, and their arrangement varies considerably.

A. _Nerves which are diverticula of the brain_ (Figs. 41–45).

The _olfactory_ nerves (_first pair_) always retain their intimate relation to the hemispheres, the ventricles of which are not rarely continued into the tubercle or even pedicle of the nerves. The different position of the olfactory tubercle has been already described as characteristic of some of the orders of fishes. In those fishes in which the tubercle is remote from the brain, the nerve which has entered the tubercle as a single stem leaves it split up into several or numerous branches, which are distributed in the nasal organ. In the other fishes it breaks up into branchlets spread into a fan-like expansion at the point, where it enters the nasal cavity. The nerve always passes out of the skull through the ethmoid.

The _optic_ nerves (_second pair_) vary in size, their strength corresponding to the size of the eye; they take their origin from the _lobi optici_, the development of which again is proportionate to that of the nerves. The mutual relation of the two nerves immediately after their origin is very characteristic of the sub-classes of fishes. In the _Cyclostomes_ they have no further connection with each other, each going to the eye of its own side.[11] In the _Teleostei_ they simply cross each other (_decussate_), so that the one starting from the right half of the brain goes to the left eye and _vice versa_. Finally, in _Palæichthyes_ the two nerves are fused together, immediately after their origin, into a _chiasma_. The nerve is cylindrical for some portion of its course, but in most fishes gradually changes this form into that of a plaited band, which is capable of separation and expansion. It enters the bulbus generally behind and above its axis. The foramen through which it leaves the skull of Teleostei is generally in a membranous portion of its anterior wall, or, where ossification has taken place, in the orbito-sphenoid.

B. _Nerves proper taking their origin from the brain_
(Figs. 41–45).

The _Nervus oculorum motorius_ (_third pair_) takes its origin from the Pedunculus cerebri, close behind the lobi inferiores; it escapes through the orbito-sphenoid, or the membrane replacing it, and is distributed to the musculi rectus superior, rectus internus, obliquus inferior, and rectus inferior. Its size corresponds to the development of the muscles of the eye. Consequently it is absent in the blind _Amblyopsis_, and the Myxinoids. In _Lepidosiren_ the nerves supplying the muscles of the eye have no independent origin, but are part of the ophthalmic division of the Trigeminus. In _Petromyzon_ these muscles are supplied partly from the Trigeminus, partly by a nerve representing the Oculo-motor and Trochlearis, which are fused into a common trunk.

The _Nervus trochlearis_ (_fourth pair_), if present with an independent origin, is always thin, taking its origin on the upper surface of the brain from the groove between lobus opticus and cerebellum; it goes to the Musculus obliquus superior of the eye.

C. _Nerves taking their origin from the Medulla oblongata_
(Figs. 41–45).

The _Nervus abducens_ (_sixth pair_) issues on the lower surface of the brain, taking its origin from the anterior pyramids of the Medulla oblongata, and supplies the Musculus rectus externus of the eye, and the muscle of the nictitating membrane of Sharks.

The _Nervus trigeminus_ (_fifth pair_) and the _Nervus facialis_ (_seventh pair_) have their origins close together, and enter into intimate connection with each other. In the Chondropterygians and most Teleostei the number of their roots is four, in the Sturgeons five, and in a few Teleostei three. When there are four, the first issues immediately below the cerebellum from the side of the Medulla oblongata; it contains motory and sensory elements for the maxillary and suspensorial muscles, and belongs exclusively to the trigeminal nerve. The second root, which generally becomes free a little above the first, supplies especially the elements for the Ramus palatinus, which sometimes unites with parts of the Trigeminal, sometimes with the Facial nerve. The third root, if present, is very small, and issues immediately in front of the acustic nerve, and supplies part of the motor elements of the facial nerve. The fourth root is much stronger, sometimes double, and its elements pass again partly into the Trigeminal, partly into the Facial nerve. On the passage of these stems through the skull (through a foramen or foramina in the alisphenoid) they form a ganglionic plexus, in which the palatine ramus and the first stem of the Trigeminus generally possess discrete ganglia. The branches which issue from the plexus and belong exclusively to the Trigeminus, supply the organs and integuments of the frontal, ophthalmic, and nasal regions, and the upper and lower jaws with their soft parts. The Facial nerve supplies the muscles of the gill-cover and suspensorium, and emits a strong branch accompanying the Meckelian cartilage to the symphysis, and another for the hyoid apparatus.

The _Nervus acusticus_ (_eighth pair_) is strong, and takes its origin immediately behind, and in contact with, the last root of the seventh pair.

The _Nervus glossopharyngeus_ (_ninth pair_)[12] takes its origin between the roots of the eighth and tenth nerves, and issues in Teleostei from the cranial cavity by a foramen of the exoccipital. In the Cyclostomes and Lepidosiren it is part of the Nervus vagus. It is distributed in the pharyngeal and lingual regions, one branch supplying the first branchial arch. After having left the cranial cavity it swells into a ganglion, which in Teleostei is always in communication with the sympathic nerve.

The _Nervus vagus_ or _pneumogastricus_ (_tenth pair_) rises in all Teleostei and Palæichthyes with two discrete strong roots: the first constantly from the swellings of the corpora restiformia, be they thinner or thicker and overlying the sinus rhomboidalis, or be they developed into lateral plaited pads, as in Acipenser and Chondropterygians. The second much thicker root rises from the lower tracts of the medulla oblongata. Both stems leave the cranial cavity by a common foramen, situated in Teleosteous fishes in the exoccipital; and form ganglionic swellings, of which those of the lower stem are the more conspicuous. The lower stem has mixed elements, motory as well as sensory, and is distributed to the muscles of the branchial arches and pharynx, the œsophagus and stomach; it sends filaments to the heart and to the air-bladder where it exists. The first (upper) stem forms the _Nervus lateralis_. This nerve, which accompanies the lateral mucous system of the trunk and tail, is either a single longitudinal stem, gradually becoming thinner behind, running superficially below the skin (Salmonidæ, Cyclopterus), or deeply between the muscles (Sharks, Chimæra), or divided into two parallel branches (most Teleostei): thus in the Perch there are two branches on each side, the superficial of which supplies the lateral line, whilst the deep-seated branch communicates with the spinal nerves and supplies the septa between the myocommas and the skin. In fishes which lack the lateral muciferous system and possess hard integuments, as the Ostracions, the lateral nerve is more or less rudimentary. It is entirely absent in Myxinoids, but the gastric branches of the Vagus are continued, united as a single nerve, along the intestine to the anus.

No fish possesses a _Nervus accessorius_. Also a separate _Nervus hypoglossus_ (_twelfth pair_)[13] is absent, but elements from the first spinal nerve are distributed in the area normally supplied by this nerve in higher vertebrates.

* * * * *

The number of _Spinal_ nerves corresponds to that of the vertebræ, through or between which they pass out. Each nerve has two roots, an anterior and posterior, the former of which has no ganglion, and exclusively contains motor elements. The posterior or dorsal has a ganglionic enlargement, and contains sensory elements only. After leaving the vertebral canal each spinal nerve usually divides into a dorsal and ventral branch. The Gadoids show that peculiarity that each of the posterior roots of some or many of the spinal nerves possesses two separate threads, each of which has a ganglion of its own; the one of these threads joins the dorsal and the other the ventral branch. In fishes in which the spinal chord is very short, as in Plectognaths, Lophius, the roots of the nerves are extremely long, forming a thick _Cauda equina_. The additional function which the (five) anterior spinal nerves of _Trigla_ have to perform in supplying the sensitive pectoral appendages and their muscles has caused the development of a paired series of globular swellings of the corresponding portion of the spinal chord. A similar structure is found in _Polynemus_.

Brain and anterior portion of the spinal chord of Trigla
(Gurnard), showing the globular swellings at the base of the
anterior spinal nerves.]

A _Sympathic nervous_ system appears to be absent in _Branchiostoma_, and has not yet been clearly made out in _Cyclostomes_. It is well developed in the _Palæichthyes_, but without cephalic portion. This latter is present in all Osseous fishes, in which communication of the Sympathic has been found to exist with all cerebral nerves, except the olfactory, optic, and acustic. The sympathic trunks run along each side of the aorta and the back of the abdomen into the hæmal canal; communicate in their course with the ventral branches of each of the spinal nerves; and, finally, often blend together into a common trunk beneath the tail. At the points of communication with the cerebral and spinal nerves frequently ganglia are developed, from which nerves emerge which are distributed to the various viscera.