618 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



primitive fibres or tubes, and composed alto- 

 gether of isolated cells, arranged in a linear 

 direction only towards the middle of the cord, 

 can transmit influences in any given direction; 

 and more especially how the tract of black or 

 grey matter, if it exercises any peculiar func- 

 tion (excito-motory) communicates with the 

 origin of the nerves. The nerves, also, are 

 remarkable, originating in single roots, and 

 containing in their composition one kind only 

 of primitive fibres (cylindrical)."] 



Fig. 351. 



a, first cerebral mass 

 or olfactory tubercle, b, 

 olfactory tubercle sliced, 

 showing its solid struc- 

 ture, c, second cerebral 

 mass or optic lobe, of large 

 relative size. d, optic 

 lobe, cut open to shew 

 the internal cavity, e, 

 tubercles in the cavity. 

 f, third cerebral mass or 

 cerebellum, tongue-sha- 

 ped, g, spinal cord, h, 

 posterior longitudinal fis- 

 sure of spinal cord, i, 

 central canal of spinal 

 ,, cord, k, olfactory nerves. 

 /, optic nerves, m, fifth 

 pair of nerves, n, acous- 

 tic nerve, o, glossopha- 

 ryngeal nerve of Cuvier. 

 p, eighth pair par va- 

 gum. q, bristle passed 

 under the cerebellum and 

 along the fourth ventricle, 

 shewing the communica- 

 tion of this latter with the 

 cavity of the optic lobes 

 or third ventricle. 



Brain and portion of spinal marrow of Gadus morhua 

 (Cod-Jish), about natural sixe. 



The brain, especially in the lowest of the 

 fishes, presents quite the appearance of a series 

 of ganglia developed on the superior surface of 

 the cords of the spinal marrow (jig. 351, , b, 

 c). In many species it is extremely small, and 

 by no means fills the cranial cavity ; in the 

 mackarel, the volume of the brain and of the 

 cavity destined to receive it are nearly equal. 

 Its very small size is at once evident by com- 

 paring; its weight with that of the whole body 

 of the animal : thus in a chub, weighing 842 

 scruples, the brain weighed only one scruple, 

 the proportions being as 100 : 84200 ; in a 

 carp, weighing 11280 grains, the brain weighed 

 only fourteen grains, the proportions being as 

 100 : 80600 ; in a roach, weighing 5030 grains, 

 the brain weighed only nine grains and a half, 

 the proportions being as 100 : 52,900 ; and, as 

 before observed, in a lamprey weighing 570 

 grains, the brain weighed only four-tenths of a 

 grain, the proportions being as 100 : 142,500. 

 [In Leuret's table a discrepancy still more 

 striking may be observed. This author gives 

 as a mean the proportion 1 : 5668.*] We thus 

 observe how small is the proportion which the 

 size of the brain bears to that of the rest of the 

 body, and consequently how imperfect is as yet 

 the de\ elopement of the encephalic mass. 



* Syatemc ncrvcux, t. i. p. 153. 



On taking a general review of the con- 

 formation of the cerebral masses forming the 

 brain of fishes, we find it to consist of a suite 

 of ganglia arranged behind each other two 

 pairs and a single one : 1st, there are two gan- 

 glia or lobes, situated the most anteriorly, the 

 olfactory lobes; immediately behind which are 

 two others, generally of larger size, the optic 

 lobes; and behind these, again, is a single 

 ganglion or lobe, situated in the median line, 

 the, cerebellum. On the inferior surface, im- 

 mediately underneath the optic lobes, are two 

 more ganglia. The names that have been given 

 to these parts are extremely various ; and re- 

 specting the relations and analogies which they 

 bear to the brain of the higher animals, great 

 difference of opinion exists. 



1st. The olfactory tubercles, orjirst cerebral 

 mass (jigs. 351 and 352, , ), which, with 

 Arasky,* Serres,f Desmoulins,| Carus, and 

 Tiedemann,|| and contrary to Collins,^! Monro,** 

 Camper,ft libel,}} Treviranus, and Cuvier,|||| 

 I consider as analogous to the cerebral hemi- 

 spheres of man, are generally of small size, and 

 contain no cavity (_fig. 351, 6). In the eel 

 they consist of three pairs of ganglia, which 

 increase in comparative size from before to 

 behind ; in the carp and mackarel, of only one 



Fig. 352. 



Brain and portion of spinal marrow of Levciscits 

 (Chub), about natural sixe. 



a, first cerebral mass or olfactory tubercle, b, 

 second cerebral mass or optic lobe, c, third cere- 

 bral mass or cerebellum, d, spinal marrow, with 

 its posterior longitudinal fissure, e, fourth ventri- 

 cle, k, olfactory nerve. /, tuberculous enlarge- 

 ment of the olfactory nerve. 



* De piscium cerehro. 



t Anatomie Comparee du Cerveau. 



| Comparative Description of the Brain in the 

 Four Classes of Vertebrated Animals. 



f> Anatomie Comp;iree. 



|| Anatomy of the Foetal Brain. This anatomist 

 considers them as more particularly " analogous 

 to the corpora striata, on the external borders of 

 which the membranous hemispheres are not yet 

 elevated." 



H System of Anatomy. 



** Anat. of Fishes. 



tt Memoir on the Ear of Fishes, 17621774. 



If Observationes Nevrologic;e ex Anatome Com- 

 parata, 1788. 



$$ Memoir on the Brain, 1817. 



|||| Lccons d'Anatomic Comparee. 



