Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/669

651 NEUROLOGY,] ICHTHYOLOGY 651 cerebri magna), may be seen. A vascular membranous Bac, epiphysis, containing lymphatic fluid, takes its origin from the third ventricle, its base being expanded over the anterior interspace of the optic lobes, and the apex being FIG. 41. Brain of Polypterui. (After MUIler.) I. Upper, II., Lateral, III. Lower aspect, a, medulla; 6, corpora restiformia; c, cerebellum; d, lobi optici; e, hypophysis; /, fissura cerebri magna; g, nervus opticus; g, chiasma; It, hemispheres ; t , lobus olfactorius ; k, sinus rhomboidalis (fourth ventricle). fixed to the cartilaginous roof of the cranium. This struc ture is not peculiar to the Ganoids, but is found in various stages of development in Teleosteans, marking, when pre sent, the boundary between the prosencephalon and mesen- cephalon. 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. As regards external configuration, the brain of Lepid- osteus 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 Chondropterygians (fig. 42) is more developed than that of other fishes, and is distinguished by well-marked cha racters. These are first, the prolonga tion of the olfactory lobes into pedicles of greater or less length, which dilate into great ganglionic masses, where they Come into Contact Fl - 42. Brain of Carcharias. (After Owen.) ac, with the olfactory 2K ^^li^^^S^, Sc^S Sacs; Secondly, tllQ j ticus ; &quot;- hemisphere; f, lobus olfactorius; i , , ., J&amp;gt; olfactory pedicle ; k, nervus olfactorius ; /, epi- Space Wllich gene- physls; m, nervus oculo-motorius ; tr nervus rally intervenes be- tri s euUnu8 : r, nervus vagus, tween the prosencephalon and mesencephalon, as in some Ganoids; thirdly, the large development of the meten cephalon. The cerebellum is very large, overlying a portion of the optic lobes and of the sinus rhomboidalis, and is often 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 or lobi nervi trigemini). The brain of the Cyclostomes 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. The foremost pair are the large olfactory tubercles, which are extremely large in Petro- myzon. 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, but less distinctly in Petromyzon. The last pair are the corpora quadrigemina. According to this in terpretation, the cerebellum would be absent in Myxinoids, and represented in Petromyzon by a narrow commissure- only, stretching over the foremost part of the sinus rhom boidalis. In the Myxinoids the medulla oblongata ends in two divergent swellings, free and obtuse at their ex tremity, from which most of the cerebral nerves take their origin. 1 Two very important conditions require mention. The first relates to the optic nerves, viz., to their mutual relation immediately after their origin, which is very characteristic of the subclasses of fishes. In the Cyclo stomes they have no further connexion with each other, each going to the eye of its own side. In the Tdeostei (fig. 40, n) 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 the Palceichthyes (fig. 41, g } the two nerves are fused together, immediately after their origin, into a chiasma. The second noteworthy peculiarity occurs in the distri bution of the nervus vagus ; it emits a strong branch, called nervus lateralis, which accompanies the lateral mucous system of the trunk and tail. This is either a single longitudinal stem, gradually becoming thinner behind, and running superficially below the skin (Sal- monidce, Cydopterits) or deeply between the muscles (sharks, Chimcera), or is divided into two parallel branches (most Teleostei] ; thus in the perch there are two branches on each side, the superficial one supplying the lateral line, whilst the deep-seated branch communicates with the spinal nerves and supplies the septa between the myocom- mas 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. Fishes possess all the spino-cerebral nerves of the higher Vertebrata, with the exception of the nervus acces- sorius. A separate nervus hypoglossus is also absent, but elements from the first spinal nerve are distributed over the area normally supplied by this nerve in higher ver tebrates. The number of spinal nerves corresponds to that of the vertebra;, through or between which they pass out. A sympathic nervous system appears to be absent in Branchiostoma, and has not yet been clearly made out in the Cyclostomes. It is well developed in the PalaicMhyes, but without the cephalic portion. This latter is present in all osseous fishes, in which the communication of sympathy has been found to exist between all the cerebral nerves, except the olfactory, optic, and acoustic. The sympathic trunks run along each side of the aorta and the back of the abdomen into the haemal 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 ganglia are frequently developed, from which nerves emerge which are distri buted to the various viscera. 1 For a more detailed account of the spino-cerebral iiurves, see In troduction to tlie Study of Fishes, by A. Giinther.