Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/668

650 650 ICHTHYOLOGY [NEUROLOGY. In Torpedo the electric organs are large, flat, uniform bodies, lying one on each side of the head, bounded behind by the scapular arch, and laterally by the anterior crescent-shaped tips of the pectoral fins. They consist of an assemblage of vertical hexagonal prisms, whose ends are in contact with the integuments above arid below ; and each prism is subdivided by delicate transverse septa, forming cells filled with a clear, trembling, jelly-like fluid, and lined within by an epithelium of nucleated corpuscles. Between this epithelium and the transverse septa and walls of the prism there is a layer of tissue on which the terminations of the nerves and vessels ramify. Hunter counted four hundred and seventy prisms in each battery of Torpedo marmorata, and demonstrated the enormous supply of nervous matter which they receive. Each organ receives one branch of the trigeminal nerve and four branches of the vagus, the former, and the three anterior branches of the latter, being each as thick as the spinal chord (electric lobes). l Mcilapterurus the electric organ extends over the whole body, but is thickest on the abdomen ; it lies between two aponeurotic membranes below the skin, and consists of rhomboidal cells which contain a rather firm gelatinous substance. The electric nerve takes its origin from the spinal chord, does not enter into connexion with ganglia, and consists of a single enormously-strong primitive fibre, which distributes its branches in the electric organ. The electric eel is the most powerful of electric fishes. Its electric organ consists of two pairs of longitudinal bodies, situated immediately below the skin, above the muscles, one pair on the back of the tail, and the other pair along the anal fin. Each fasciculus is composed of flat partitions or septa, with transverse divisions between them. The outer edges of the septa appear in nearly parallel lines in the direction of the longitudinal axis of the body, and consist of thin membranes, which are _easily - torn ; they serve the same purpose as the columns in the analogous organ of the torpedo, making the walls or abutments for the perpendicular and transverse dissepi ments, which are exceedingly numerous, and so closely aggregated as to seem almost in contact. The minute prismatic cells, intercepted between these two sorts, of plates, contain a gelatinous matter ; the septa are about -^5-th of an inch from each other, and a length of one inch contains a series of two hundred and forty cells, giving an enormous surface to the electric organs. The whole apparatus is supplied with more than two hundred nerves, which are the continuations of the rami anteriores of the spinal nerves. In their course they give out branches to the muscles of the back, and to the skin of the animal. In the Gymnotes, as in the torpedo, the nerves supplying the electric organs are much larger than those bestowed on any part for the purposes of sensation or movement. The. phenomena attending the exercise of this extra ordinary faculty closely resemble muscular action. The time and strength of the discharge are entirely under the control of the fish. The power is exhausted after some time, and needs repose and nourishment to restore it. If the electric nerves are cut and divided from the brain, the cerebral action is interrupted, and no irritant to the body has any effect in exciting electric discharge ; but if their ends be irritated the discharge takes place, just as a muscle is excited to contraction under similar circumstances. Singularly enough, also, the application of strychnine causes simultaneously a tetanic state of the muscles and a rapid succession of involuntary electric discharges. The strength of the discharges depends entirely on the size, health, and energy of the fish, an observation completely agreeing with that made on the efficacy of snake-poison. Like this latter, the property of the electric force serves two ends in the economy of the animals which are endowed with it : it is necessary to them for overpowering, stunning, or killing the creatures on which they feed, whilst incidentally they use it as the means of defending themselves from their enemies. NEUROLOGY, The most simple condition of the nervous central organ known in vertebrates is found in Branchiostoma. In this fish the spinal cord tapers at both ends, no anterior cerebral swelling, or anything approaching a brain, being present. 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 iuto a dorsal and a 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 cord of the Cyclostomes is flattened in its whole extent, band- like, and elastic ; also in Chimera it is elastic, but flattened in its posterior portion only. In all other fishes it is cylindrical, non-ductile, and generally extend ing along the whole length of the spinal canal. The Plectognaths offer a singular exception in this respect, that the spinal cord is much shortened, the posterior portion of the canal being occupied by a long cauda equina ; this shortening of the spinal cord has become extreme i n the sun-fish (Orthagoriscus), in which it has shrunk into a short and conical appendage of the brain. In the devil-fish (Lophius) also a long cauda equina partly con ceals the cord 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 the y^jth part of the weight of the entire fish, in the pike the y^^th part, and in the large sharks it is relatively still smaller. o. 40. Brain of Perch. I. Upper aspect. II. Lower aspect, a, cerebellum; 6, optic lobes; c, hemispheres; e, lobi Interferes; /, hypophysis; 9, It bi posteriores ; i, olfactory lobes; n, :iervus opticus; o, nervus olfactorius; p, nervus oculo-motorius ; q, nervua trochlearis ; r, nervus triReminus ; s, nervus acousticus ; t, nervus vagus ;, nervua abducens; v, fouith ventricle. The brain of osseous fishes (fig. 40) viewed from above shows three protuberances, respectively termed the prosen- cephalon, 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. The optic lobes possess a cavity (ventriculus lobi optici), 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 cord, and contains the fourth ventricle. 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). The brain of Ganoid fishes shows great similarity to that of the Teleostei ; there is, however, considerable diversity in the arrangement of its various portions in the different types. In the sturgeons and Polypterus (fig. 41) the hemispheres are more or less remote from the mesen cephalon, so that in an upper view the crura cerebri, with the intermediate entrance into the third ventricle (fissura