MAMMALIA 331 



comes folded into deep pockets and gives to the tooth a complex 

 cross-section, as in the ungulates. The tooth remains hollow, a pulp 

 cavity remaining in its center, which contains blood-vessels, nerves, 

 and connective tissue. The dentine is merely a fine quality of bone 

 and has the histological structure of the latter. 



The Mammalian Brain. — Although the brains of certain archaic 

 mammals were not much more highly developed than those of some 

 of the reptiles, those of modern mammals, especially those of the more 

 highly specialized groups, show marked advances over the brains of 

 other vertebrates. The mammal brain (Fig. 172) is relatively large, 

 but the cerebral hemispheres show more increase than do other parts. 

 These hemispheres are connected by an elaborate system of commis- 

 sures, which serve to correlate the two and to make them act as one 

 organ; the corpus callosum is the most important of these commis- 

 sures and it reaches a large size in the highest mammals. The surface 

 of the cerebrum, in all but the more primitive mammals, is much in- 

 folded into a system of convolutions, which greatly increase the sur- 

 face without unduly increasing its bulk. It is not strictly true that 

 the degree of complexity of the cerebral convolutions is an index of 

 the grade of intelligence; for the elephant has the most elaborately 

 convoluted cerebrum, but is hardly as intelligent as many other 

 mammals with less convolutions. The optic lobes are four in num- 

 ber, but in size they are small. The cerebellum is scarcely as elaborate 

 as in the birds, though better developed than in any reptile. 



Urogenital Systems of Mammals. — The kidneys are compact in 

 form and are of the metanephros type. They are usually asym- 

 metrical in position, one lying more anteriorly than the other. The 

 ureters lead directly to the urinary bladder, which is formed out of 

 the remains of the allantois. 



The ovaries are always paired; never single as in the bird. They 

 are very small in size, since they produce minute eggs with little or 

 no yolk. This small size of ovaries and eggs is in correlation with 

 the habit of uterine gestation. The paired oviducts enlarge into 

 paired uteri, which in some groups unite into a single median 

 uterus. 



The testes lie at first in the body cavity, as in reptiles, and occupy 

 positions homologous with those of the ovaries. In most mam- 

 mals (monotremes, whales, elephants, armadillos, and a few others 

 excepted) the testes descend through the gubernaculum into the 



