tab. iv.): a full and accurate detail of the myology of the Myxinoids, together with a philosophical comparison of the muscular system of Fishes generally with that of the higher Vertebrata, will be found in XXI. pp. 179-246. But the determination of the special, serial, and general homologies, and the recognition of the various individual adaptive modifications, of the muscles of Fishes, still remain a rich and little-explored field for the labours of the myologist.

The normal character of Ichthyic myology shows itself in the vast proportion of the vegetatively repeated myocommata, corresponding with the vertebral arches, as compared with the superadded system of muscles subservient to the action of their diverging appendages: but this condition, which, inasmuch as it deviates so little from the fundamental type, throws so much light upon the essential nature and homologies of the muscles of the Vertebrata, is not less admirably and expressly adapted to the habits and medium of existence of the Fish. The interlocked myocommata of the trunk constitute, physiologically, two great lateral muscular masses, adapted by their attachments, and especially by those of the anterior and posterior ends, to bend vigorously from side to side, with the whole force of their alternating antagonistic contractions, the caudal moiety of the trunk; producing that double lash of the tail by which the fish darts forwards with such velocity. When the lateral muscles are more violently contracted, so as to bend the whole trunk, the recoil may even raise and propel the fish some distance from its native element : thus the salmon overleaps the roaring cataract which opposes its migration to the shallow sources whither an irresistible instinct impels it to the business of spawning; and thus the flying-fish, in the extremity of danger, baffles its pursuer by springing aloft, and prolongs its oblique course through the air by the rapid fluttering of its outspread pectorals. When the anterior portions of the great lateral masses act from the trunk as a fixed point upon the head, they move it rapidly and forcibly from side to side: in this way the Siluri deal severe blows with their outstretched serrated pectoral spines; thus the Percoid and Cottoid Fishes strike with their opercular spines; and so likewise may the Saw-fish (Pristis), and Sword-fish (Xiphias), wield their formidable weapons, although their deadly cut or thrust is commonly delivered with the whole impetus of the onward course, the head being rigidly fixed upon the trunk.

The supra-carinales, combining with the dorsal portions of the myocommata, give tension to the region of the back, slightly raise the tail, and depress the dorsal fins. The infra-carinales, in combi

nation with the retractores pubis, tend to compress the abdomen, to constrict the anus, and to depress the tail.

The muscles of the pectoral fins, though, compared with those of the homologous members in higher vertebrates, they are very small, few, and simple, yet suffice for all the requisite movements of the fins; elevating, depressing, advancing, and again laying them prone and flat, by an oblique stroke, upon the sides of the body. The rays or digits of both pectorals and ventrals, as well as those of the median fins, can be divaricated and approximated, the intervening webs spread out or folded up, and the extent of surface required to react upon the ambient medium in each change and degree of motion, can be duly regulated at pleasure.

LECTURE VIII.

NERVOUS SYSTEM OF FISHES.

THE neural axis is a simple continuous chord in the Lancelet (Branchiostoma, fig. 46, md.), of opaline sub-transparency, ductile

*

and elastic, flattened, composed entirely of nucleated cells showing a feeble indication of a median linear arrangement, which becomes more general and distinct at the anterior end, where the axis becomes cylindrical and terminates obtusely: the nerves, trigeminal (ob), and optic (op), in connection with this slightly modified part of the axis, indicate it to be the brain. This is the most simple persistent condition of the central organs of the nervous system known in the vertebrate sub-kingdom: it is typified by that of the Entozoa in the articulate sub-kingdom. In all other Fishes the fore part of the neural axis receives the vagal, trigeminal, and special-sense nerves,

*Prof. Goodsir, lxxxix.

and developes and supports ganglionic masses, principally disposed in a linear series parallel with the axis: this part is called the brain' or encephalon: the rest of the axis I term the 'myelon'*; retaining its columnar or chord-character, and, being lodged in the canal of the spinal column, it is usually defined as the medulla spinalis, spinal marrow, or spinal chord.

In the Lamprey the myelon is flattened, opaline, ductile, and elastic, as in the Lancelet and other Dermopteri: in typical Fishes it is inelastic and opaque, cylindrical or sub-depressed, of nearly uniform diameter, gradually tapering in the caudal region to a point in heterocercal Fishes, but swelling again into a small terminal ganglion† in most homocercal Fishes.

The Hunterian preparation of the skate (Raia Batis, No. 1347.) shows a slight (brachial or pectoral) enlargement of the myelon where the numerous large nerves are sent off to the great pectoral fins a feebler brachial enlargement may be noticed in the Sharks. I have not recognised it in osseous Fishes; not even in those with enormous pectorals adapted for flight, e. g. Exocatus and Dactylopterus in the latter the small ganglionic risings upon the dorsal columns of the cervical region of the myelon receive nerves of sensation from the free soft rays of the pectorals, and the homologous ganglions are more marked in other Gurnards (Trigla), which have from three to five and sometimes six pairs, e. g. in Trigla Adriatica. § Similar myelonal cervical ganglions are present, also, in Polynemus. In the heterocercal Sturgeon there is a feeble expansion of the myelon at the beginning of the caudal region, whence it is continued, gradually diminishing to a point along the neural canal in the upper lobe of the tail. In some bony fishes (Trout, Blenny), the caudal ganglion is not quite terminal, and is less marked than in the Cod or Bream, in which it is of a hard texture, but receives the last

6

6

*Gr. μveλos, marrow. As an apology for proposing a name, capable of being inflected adjectively, for a most important part of the body which has hitherto received none, I may observe that, so long as the brief definitions, marrow of the spine,' chord of the spine,' are substituted for a proper name, all propositions respecting it must continue to be periphrastic, e. g. 'diseases of the spinal marrow,' functions of the spinal chord,' instead of 'myelonal diseases,' 'myelonal functions: ' or, if the pathologist speaks of spinal disease,' meaning disease of the spinal marrow, he is liable to be misunderstood as referring to disease of the spinal or vertebral column. But, were the Anatomist to speak of the canal in the spinal marrow of Fishes as the myelonal canal,' he would at once distinguish it from the canal of the spinal column. The generally accepted term 'chorda,' or 'chorda dorsalis,' for the embryonic gelatinous basis of the spine, adds another source of confusion likely to arise from the use of the term 'spinal chord,' applied to the myelon, or albuminous contents of the spinal canal.

† LIII. p 6.; LIV. p. 26. (in the Cod).

This structure is accurately figured by Mr. Swan in LIV. pl. xi.

§ Lv. pl. 2. fig. 4. p. 106.; and L. p. 6., pl. 2. fig. 24, 25.

pair of spinal nerves. The absence of this ganglion in the Shark shows that it relates not to the strength of the tail but to its form, as

47

M

depending on the concentration and coalescence of the terminal vertebræ; except, indeed, where such metamorphosis is extreme, as, e. g. in Orthagoriscus mola, and where it affects the entire condition of the myelon, which has shrunk into a short, conical, and, according to Arsaki (LIII. tab. iii. fig. 10.), gangliated appendage to the encephalon. A like singular modification, but without the ganglionic structure, obtains in Tetrodon and Diodon, in a species of which latter genus I found the myelon (fig. 47. M.) only four lines long in a fish of seven inches in length, and measuring three inches across the head. The neural canal in these Plectognathic fishes is chiefly occupied by a long 'cauda equina' (ib. c. e.). But, insignificant as the myelon here seems, it is something more than merely unresolved nerve fibres: transverse white striæ are disBrain and Mye- cernible in it, with grey matter, showing it to be a centre of nervous force, not a mere conductor. In the Lophius a long cauda equina partly conceals a short myelon which terminates in a point at about the twelfth vertebra: in other fishes the myelon is very nearly or quite co-extensive with the neural canal, and there is no cauda equina, or bundle of nerve roots, in the canal: a tendinous thread sometimes ties the terminal ganglion to the end of the canal.

lon, Diodon. nat. size.

A shallow longitudinal fissure divides the ventral surface, and a deeper one the dorsal surface, of the myelon, into equal moieties : a feeble longitudinal lateral impression (Sturgeon) subdivides these into dorsal and ventral columns: in other fishes (Cod, Herring) these are separated by a lateral tract, and six columns or chords may be distinguished in the myelon; two dorsal or sensory, two ventral or motory; and two lateral or restiform tracts. A minute cylindrical canal extends from the fourth ventricle, beneath (ventrad of) the bottom of the dorsal fissure, along the entire myelon; this canal is not exposed in the recent fish by merely divaricating the dorsal columns. Both lateral halves of the myelon have grey matter in their interior, and white transverse striæ. Although many fishes (Bream, Dorsk) show a slight enlargement at each junction of the nerve roots with the myelon, the anatomical student will look in vain in the recent Eel, or Lump-fish, for that ganglionic structure of the myelon which the descriptions of Cuvier* might lead him to expect.

XXIII. i. p. 323.; XIII. iii. p. 176.

Section of medulla

charias.

As the myelon approaches the encephalon it expands, and the following changes may be here observed, in the Cod and Shark: in the ventral columns a short longitudinal groove divides a narrower median pre-pyramidal' tract (fig. 48. a), from a broader lateral 'olivary' tract (ib. b): in the dorsal columns a median 'funicular' tract (ib. e), is similarly marked off from a lateral 'post-pyramidal' tract (d); this is now, also, distinguished by a deeper fissure from the true lateral or 'restiform' tract (c), oblongata, Car- at the inferior part of which a distinct slender portion is also sometimes defined. The post-pyramidal tracts diverge, expand and blend anteriorly with the similarly bulging restiform tracts, forming the side-walls of a triangular or rhomboidal cavity, called the fourth ventricle: the pre-pyramidal and olivary tracts forming the floor of the ventricle, are covered below by a thin superficial layer of transverse arciform fibres'* (ib. m) concealing their boundary fissures. At the bottom of the ventricle the myelonal canal is exposed, and its sides swell and rise as rounded or 'teretial' tracts (ib. f)† from the floor of the ventricle, diverging slightly as they advance, and exposing an intermediate nodular' tract; this structure is well seen in the Sturgeon and Selache: two lateral prominent 'vagal' columns, also, project inwards into the ventricle, from the conjoined restiform and post-pyramidal tracts; these vagal columns present a series of nodules, corresponding with the fasciculi of the roots of the great vagal nerve in Selache: (Prep. 1311 A).

ૐ

In the Cyprinoid fishes the median inferior tract rises into the ventricle, and is developed into a smooth hemispheric mass, the nodulus' (fig. 51. k): the conjoined post-pyramidal and restiform walls swell outwards, and form large lateral vagal' lobes (fig. 51. h) these are remarkably developed, and are nodulated in the Carp, which is so tenacious of life. The vagal lobes are enormously developed in the Torpedo; they join the trigeminal lobes, and present a yellowish colour in the recent fish: many non-nucleated cells are present in their substance; they give origin to the nerves of the electric organs, and have been called 'lobi electrici'; but the vagal lobes are scarcely less remarkable for their size in the Gymnotus, where they have no direct connection with any of the nerves of the electric organs. In the Cod the vagal ganglions are obsolete, and

* Homologous with the "filamenti arciformi" of Rolando, LVIII. p. 170. t. i. fig. 2.

†Three are called "vordere pyramiden" by Dr. Stannius, LVI. p. 43.