NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



623 



preceding classes) analogous to the optic tha- 

 lami of the brain of man : between them is the 

 canal leading to the infundibulum. 



3d. The cerebellum, or third cerebral mass, 

 (fig. 355, c), is particularly well developed, 

 exhibiting an amazingly increased degree of 

 organization when compared with the preced- 

 ing classes, and bearing great analogy to the 

 cerebellum of the higher animals. In the be- 

 fore-mentioned species it consists of a more or 

 less rounded median lobe, with very small 

 lateral appendages; its external surface is 

 marked by transverse sulci, varying in number, 

 that extend a short distance into the interior of 

 its substance. On cutting into it in the sea- 

 gull, fowl, and pigeon, the appearance of the 

 arbor vitas is slightly perceptible. In the em- 

 bryo of the chick on the sixteenth day, however, 

 the cerebellum is very small, not yet suffici- 

 ently developed to separate the optic lobes; 

 very slight traces only of grooves were ap- 

 parent on its surface (Jig. 356, c). On the 

 twentieth day it presents all the characters of the 

 full-grown bird, both as regards relative size, 

 position, and external striae (Jig. 357, c 1 ). 



Fig. 358. 



its weight, too, when compared with the body, 

 is greater; and the ganglia composing it are 

 more above, and less behind each other. The 

 primary cerebral mass has now acquired so 

 high a degree of developement as to surpass 

 the others in size ; no convolutions are, how- 

 ever, yet apparent on its surface ; no large 

 commissure yet exists to unite them. The 

 optic lobes, or median cerebral mass, are small, 

 separated from each other, and their cavities 

 have decreased. The cerebellum, or third 

 cerebral mass, is large ; traces of lateral lobes 

 are evident, and external striae are perceptible. 



MAMMALIA. In the last and highest class, 

 the Mammalia, will be found some most in- 

 teresting grades of developement and structural 

 forms of the cerebral mass to arrest our atten- 

 tion, and we shall observe how rapidly the 

 different parts are added, and those already 

 formed are more highly developed, to consti- 

 tute the complex brain of the human species. 

 The spinal cord (fgs. 358, 321, 322, 323, d*) 

 is of still less relative size than in the pre- 

 ceding classes ; it has an anterior and pos- 

 terior longitudinal fissure. In a full-grown 

 mouse, weighing 227 grains, the spinal 

 marrow weighed one grain and a half, the 

 brain six grains and a half the proportions 

 being as 100:22. We thus observe that the 

 former is of much less relative size than the 

 latter. 



The following is a table shewing the relative 

 proportions of the brain and spinal marrow in 

 the four classes of Vertebrata : 



PISCES .... 

 REPTILIA . . 

 AVES 



MAMMALIA 



Brain. 



Lamprey. ... as 100 



Triton as 100 



Pigeon as 100 



Mouse . ... as 1 00 



Spinal 

 Marrow. 

 750 

 180 

 30 

 ; 22 



Brain and portion of spinal cord of Lepus cuniculvs 

 (rabbit), natural size, left hemisphere sliced. 



a, first cerebral mass or cerebral hemispheres, 

 slightly grooved on the external surface, b, corpus 

 caflosum, short, c, cavity of lateral ventricle, d, 

 portion of corpus striatum. e, t<tnia semicircu- 

 laris. f, hippocampus major, the superior surface 

 sliced, q, masses or ganglia of the olfactory nerves, 

 with their cavities, h, bristle passed, shewing the 

 communication between the cavity of the olfactory 

 nerve and the lateral ventricle, k, second cere- 

 bral mass or tubercula quadrigemina, the anterior 

 pair the largest. I, I, third cerebral mass or cere- 

 bellum, very much grooved, d*, spinal cord, with 

 its posterior longitudinal fissure. 



On reviewing these statements of the ner- 

 vous system in the birds, we observe that the 

 brain and spinal marrow are no longer situated 

 on the same horizontal plane, and that the 

 preponderance is now in favour of the brain : 



The spinal cord passes lower down the ver- 

 tebral column than in man, but terminates by 

 a true cauda equina, as in the bat and mouse, 

 in which latter animal it is continued into the 

 sacrum, but not into the caudal vertebrae, as 

 in the preceding classes. In the bat the 

 spinal cord descends no lower than the eleventh 

 dorsal vertebra, a conformation rather un- 

 usual :* the fissure on its posterior surface was 

 deep in those animals, but it becomes less evi- 

 dent as we approach the human species. It 

 presents three distinct enlargements in its 

 course : a superior one, the medulla oblongata; 

 a median, and a posterior one, where the nerves 

 for the extremities are given off: this is the 

 case in the mouse and bat, though in the for- 

 mer animal the superior and median enlarge- 

 ments are so closely approximated as to render 

 the spinal cord of great thickness in the tho- 

 racic region of the body.f 



The following is a table shewing the relative 

 proportions of the body and brain in the four 

 classes of Vertebrata: 



* Meckel (Archiv. fur Physiologic) also states, 

 that in the hedgehog the spinal cord terminated in 

 the thoracic vertebrae. 



t Carus remarks the same thing in many of the 

 Mammalia with a short neck ; the Rodeutia, for 

 instance. 



