624 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



PISCES , 



REPTILIA. 



f Carp , 



Brain, 



, as 100 



Chub as 100 



\ Roach as 100 



vLamprey ... as 100 



\ Triton as 100 



((Turtle as 100 



AVES Pigeon.... as 100 



f Sheep as 100 



7 Pirr 



Body. 

 80,600 

 84,200 

 52,900 

 142,500 

 27,300 

 454,500 

 9,100 

 22,600 

 32,300 

 3,500 



MAMMALIA < Pig ......... as 100 



C. Mouse as 100 



In the brain of the Mammalia we shall find 

 the same parts as heretofore to occupy our 

 attention, though at an extraordinarily in- 

 creased degree of developement: this, however, 

 varying greatly in the different orders. Its 

 direction, with regard to the spinal marrow, 

 is no longer horizontal, as we found in the 

 fishes and reptiles, but approaching more or 

 less to a right angle ; the first traces of which 

 inflection were perceptible in the birds. In 

 the bat (Vespertilio inurinus); mouse (Man 

 musculus); rat (Mm rattus); rabbit ( Lepus 

 cuniculus, Jig. 358); pig (Sus scrofa domes- 

 tica); horse (Et/uits caballus) ; ass (Equus 

 minus); sheep (OvisummonJ ; deer(Cervus 

 damu); mole (Talpa Europtea); stoat (Mtts- 

 tela euninea) ; cat (Fells cutus) ; and monkey 

 (Cullithrix - 1 fig. 359) ; the brain exactly 

 fills the cranial cavity, that cavity correspond- 

 ing with the shape and size of the head. The 

 size and bulk of the brain are greater than in 

 any of the preceding classes, as shown by its 

 relative weight compared with the body. In 

 a sheep weighing, as near as could be calcu- 

 lated, 7466 drachms, the brain weighed 33 

 drachms ; the proportion of the brain to the 

 body being as 100:22600. In a pig weighing 

 about 7116 drachms, the brain weighed 22 

 drachms; the proportion being as 100:32350. 

 The brain of a horse weighed 1.56 drachms. 

 In a mouse weighing 327 grains, the brain 

 weighed 6 grains, the proportions being as 

 100:3,500. 



On taking a review of the structure of the 

 brain in Mammalia, we find that it presents a 

 great variety of form and developement in its 

 different parts. 1. The cerebral hemispheres, 

 or first cerebral mass, which vary greatly in 

 their size and extent, and are united in the 

 median line by a commissure, the corpus cal- 

 losum. 2. The optic lobes, or second cerebral 

 mass, which are here small and divided into 

 two pairs, presenting more particularly the 

 characters of the tubercula quadrigemina in 

 the human brain, under which name they will 

 in future be noticed. 3. The cerebellum, or 

 third cerebral mass, which is greatly increased 

 in developement, and presents a division into 

 median and lateral portions. 



1st. The cerebral hemispheres, or first ce- 

 rebral muss (figs. 357, 353, a) are of large 

 size, but this varies according to the order 

 in which they are examined. In the lower 

 ones they resemble very much the same parts 

 in birds, with regard to their small size 

 and their want of convolutions. In the dol- 

 phin they are very short and broad ; in the 

 ornithorynchus they are oval, and narrowed 



anteriorly. In both these animals their sur- 

 faces are smooth and unconvoluted. The same 

 occurs in the opossum and myrmecophaga di- 

 dactyla, amongst the Marsupialia. In the bat 

 they are no longer than wide (2f lines each 

 way), leaving the tubercula quadrigemina 

 quite exposed ; they are of a triangular form, 

 and perfectly smooth on their surface. In the 

 rabbit (fig. 358, a), rat, and mouse, rodent 

 animals, they are oblong ovate, but much nar- 

 rowed anteriorly. The tubercula quadrigemina 

 are quite exposed, but scarcely so much so as in 

 the bat ; their surfaces are smooth and uncon- 

 voluted, though in the rabbit there are a few 

 slight furrows ; on their inferior surface there 

 is a faint groove, dividing them into lobes, the 

 rudiments of the fissura Sylvii. In the pig, 

 horse, ass, sheep, and deer, the hemispheres 

 are more oval in form, more convex, and less 

 narrowed anteriorly ; they extend backwards, so 

 as quite to cover the tubercula quadrigemina, 

 and their surfaces are marked with numerous 

 convolutions; the fissures of Sylvius are more 

 strongly marked, and the division into lobes is 

 more apparent. In the stoat and cat they are 

 similarly shaped and convoluted on their surface, 

 and they extend backwards, covering the tuber- 

 cula quadrigemina and a portion of the cerebel- 

 lum. In the monkey (fig. 359, a) they are more 



Fig. 359. 



? (Monkey), natural 



Brain of Callithrl.r 



size, right lateral ventricle exposed. 



a, First cerebral mass or cerebral hemisphere, 

 elevated and broad, and extending backwards, 

 covering the cerebellum. *, posterior lobe of ce- 

 rebrum, free from convolutions, b, corpus callo- 

 sum. c, cavity of lateral ventricle, d, portion 

 of corpus striatum. e, t<enia semicirculuris. I, 

 third cerebral mass or cerebellum, d*, spinal 

 cord. 



rounded, very much elevated, broader in the 

 middle, and extend backwaids, covering the 

 cerebellum. The convolutions are more nume- 



