xxviii THE ANIMAL KINGDOM. 



When we rise to the vertebrates we meet with a form of nervous system quite 

 different from that of any adult invertebrate animal. In all the vertebrates which 

 have a definite skull — and this only excludes the lancelet and the aseidians — the 

 brain is a series of close-set ganglia, forming a mass situated in the skull, with definite 

 relations to the sense-organs, and tlie spinal cord is situated above the vertebral col- 

 umn, passing through the spinal canal, which is formed by the contiguous posterior 

 arches of the several vertebrie composing the spinal, or vertebral column. 



While the nervous system of all skulled vertebrates has a definite persistent situa- 

 tion, and with a similar cellular structure, there is a great difference between the brain 

 of the fishes and that of mammals, including man. In the fishes the brain cavity is 

 small compared with the size of the head, the brain being small, and there is a marked 

 equality in the size of the different lobes forming the brain, the optic lobes being 

 larger than the cerebral. In amphibians, such as the frog and toad, the brain is 

 more like that of fishes than of reptiles, but the optic lobes are a little smaller than 

 the cerebral, while the cerebellum is smaller than in many fishes. In the reptiles, as 

 seen in snakes, turtles, and crocodiles, the cerebral lobes begin to enlarge, and exceed 

 in size the optic lobes. Here the ventricle or cavity of the cerebral lobes is larger 

 than in the fishes, and the rounded eminence projecting from its anterior and inner 

 surface, called the ' corpus striatum,' is present for the first time. 



In birds the brain cavity is much larger than in any of the foregoing classes of 

 vertebrates, and the cerebral hemispheres are now greatly increased in f^i/.e, so as to 

 partly cover the optic lobes. The ciTebi'llum is also much larger than before, and 

 it is transversely ci-eased. 



Passing from the birds to mammals, there is seen to be a great advance in the form 

 of the brain of the latter animals. The brain cavity is much larger, and this is for the 

 most part occupied by two portions, the cerebrum and the cerebellum. The cerebral 

 hemispheres entirely conceal from above tlie olfactory and optic lobes, the surface is 

 convoluted, while behind it either touches or overlaps, so as in man to completely 

 conceal the cerebellum. The cerebral hemispheres, then, form the back of the mam- 

 malian brain, and the higher orders are usually characterized by an increase both in 

 the size of the cerebr.al hemispheres, and as a rule, though there are exceptions noted 

 farther on, in the number and complexity of the convolutions of the surface. Thus in 

 the highest mannnals, especially the gorilla and man, the increase<l size of the biain 

 in pro]iortion to the greater bulk of the body is very marked. 



Leuret has ap])roximately shown the average ]>roportional weight of the brain to 

 the body, in four classes of vertebrates, as follows : in Fishes, as 1 to 5,668 ; in rep- 

 tiles, as 1 to 1,321 ; in birds, as 1 to 212 ; in Mammalia, as 1 to 186. The brain is, 

 however, subject to the same laws as other parts of the body. There is in no organ a 

 regular and continuous progressive increase in size and comjilexity in any class of the 

 animal kingdom. The size of the cerebral hemisphere differs in different monkeys, and, 

 as has been remarked by Bastian, in the higher types of lower orders the brain is often 

 better developed than among the lower types of higher orders. Thus in the Midas 

 marmoset the convolutions are absent, so that in this respect this jirimate is on a level 

 with the monotremes and lower marsupials and rodents. In dwarf or small-sized 

 membei'S of a grou]:) the bi-ain is larger in proportion to the body than in the full-sized 

 members. Thus among marsupials, as Owen states, the size of the brain of the jiigniy 

 petaurist is to the size of the body as 1 to 25, while in the great kangaroo it is as 1 to 

 800 ; among rodents it is as 1 to 20 in the harvest mouse, but is a.s 1 to 300 in the 



