NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



G20 



the mass of nervous matter, it is greatly in 

 favour of the spinal marrow, though, as regards 

 complexity of structure, the brain preponde- 

 rates. Again, the extreme smallness of this 

 latter compared to the rest of the body, the 

 simple formation of the different masses com- 

 posing it, and the predominance of the median 

 one, (which in the lower animals is the only 

 one developed,) are points that also mark its 

 low degree of developement. Still the ground- 

 work of the most important structures has been 

 laid, and we shall trace these identical parts in 

 the succeeding classes of animals through vari- 

 ous modifications of form and phases of deve- 

 lopement. 



2 and 3. AMPHIBIA AND REPTILIA. We 

 now proceed to the Amphibia, the Batrachia of 

 Cuvier, which, in a system of arrangement, 

 must be considered as a class distinct from the 

 true Reptilia ; but their nervous system pre- 

 senting so great a similarity in structure and 

 conformation to that class, and, indeed, differ- 

 ing only in an inferiority of developement, we 

 will, to save time and space, notice the two 

 classes of Amphibia and true Reptilia con- 

 jointly. The nervous system in these animals 

 bears a great similarity in structure and deve- 

 lopement to the fishes. 



The spinal cord presents much the same cha- 

 racter as in the class just described, with regard 

 to its relative size, its extent, (excepting in the 

 frog,) and its physical conformation. In a 

 species of Triton weighing 39 grains, the spinal 

 marrow weighed | grain, and the brain only j 

 gram, the proportion being as 100 to 180. We 

 thus observe that the weight of the spinal mar- 

 row preponderates over that of the brain, al- 

 though not to so great an extent as in the fishes, 

 in consequence of the increased developement 

 of the latter. In most of the Amphibia, and 

 in all the Reptilia, the spinal cord passes down 

 the whole length of the caudal vertebrae, as in 

 the fishes, but to this the frog forms an excep- 

 tion. In that animal it descends no lower in 

 its adult state than barely midway between the 

 anterior and posterior extremities, and termi- 

 nates by a few nervous filaments, which pass 

 downwards towards the sacrum ; in the young 

 and tadpole state, however, it is prolonged 

 into the coccygeal vertebra, and terminates in 

 a point. The form and structure of the spinal 

 cord, and of the medulla oblongata, differ but 

 Jittle from what has been described in the 

 fishes. In the triton and frog there is a lon- 

 gitudinal fissure on its anterior and posterior 

 aspects and a central canal communicating with 

 the cavity of the fourth ventricle which is very 

 large, covered over by a vascular plexus, is 

 formed in the same manner, and bears great 

 resemblance to the fourth ventricle described 

 in the lamprey : in the lumbar region the 

 spinal cord is thickened where the nerves of 

 the extremities are given off; in the tadpole state, 

 however, no such enlargement is visible. 

 Amongst the true Reptilia, in the ringed snake 

 (Coluber natrix), lizard ( Lacerta viridisj, and 

 turtle ( Testudo my das, Jig. 353), the spinal cord 

 has an anterior and posterior longitudinal fis- 

 sure, and a ccnlrul canal (5) communicating 



with the fourth ventricle (A), which in the 

 ringed snake and lizard is small, but deep ; in 

 the turtle, large, but shallow, and partly co- 

 vered in by the cerebellum. According to 

 Bojanus,* the spinal cord in the Chelonia be- 

 comes enlarged where the nerves for the ante- 

 rior and posterior extremities are given off, and 

 very thin between those enlargements. Carusf 

 has observed the same enlargements, but in a 

 less degree, in a young crocodile. 



The brain is composed of a suite of ganglia 

 approaching very much in form and character 

 to the fishes, especially the Rays and Sharks. 

 In the triton (Triton cristata), frog (Rana 

 temporaria), viper (Coluber verus), ringed 

 snake (Coluber natrix), lizard (Lacerta viridis, 

 fig. 354), and turtle (Testudo myda.s,fg. 353), 

 Fig. 353. ttf first cere b ra l mass 



or cerebral hemispheres. 

 b*, first cerebral mass cut 

 open, shewing its inter- 

 nal cavity and tubercle. 

 b, second cerebral mass 

 or optic lobe, c*, second 

 cerebral mass cut open, 

 shewing the small inter- 

 nal cavity, c, third cere- 

 bral mass or cerebellum, 

 turned slightly upwards. 

 f, posterior longitudinal 

 fissure of spinal cord, g, 

 'If central canal of spinal 

 cord, h, fourth ventri- 

 cle, t, olfactory nerve. 

 h, bulbous enlargement 

 at the origin of the olfac- 

 tory nerve cut open,she w- 

 ing its internal cavity. 

 I, bristle shewing the 

 communication between 

 the cavity of the olfactory 

 nerve and the cerebral 

 hemisphere, m, bristle 

 shewing the communica- 

 tion between the cavity 

 of the optic lobe and the 

 fourth ventricle, n, bris- 

 tle passed along the cen- 

 tral canal of the spinal 

 cord, o, bristle passed 

 under the cerebellum to 

 raise it upwards, to shew 

 the fourth ventricle more 

 distinctly. 



Brain and portion, of spimd marrow of Testudo myda 

 ( turtle), about natural size. 



it fills the cranial cavity destined to receive it, 

 though that cavity is very small when compared 

 with the whole head ; thus the size of the head 

 is no criterion for the size of the brain. Its 

 weight, too, when compared with the body, is 

 another proof of its small size. In a turtle 

 weighing upwards of 50 pounds, the brain 

 (with the olfactory nerves, and a very small 

 portion of the spinal marrow), weighed only 

 77 grains, the proportions being as 100: 454,500; 

 and, as before observed, in a triton weighing 39 

 grains, the brain weighed only } grain, the pro- 

 portions being as 100 : 27,300. 



On taking a general review of its structure, 

 we find, as before, three principal parts to oc- 



* Anatome testudinis European, 

 t Op. cit. vol. i. p. 78. 



