624 CHORDATA. 



the two sides of the cerebellum. These connexions in the other 

 vertebrates are small, and even in the lower mammals, like mono- 

 tremes and marsupials, are but slightly developed. 



The increase of cerebrum and cerebellum, which occurs chiefly in the 

 dorsal portion, has resulted in flexures in the axis of the brain, already in- 

 dicated in the reptiles, increased in the birds, and reaching their maximum 

 in the mammals. Instead of continuing in the course of the spinal cord, 

 the axis of the brain bends ventrally in the medullar region (cervical 

 flexure), then in the region of the pons again dorsally (pontal flexure), an 

 at the level of the optic lobes again ventrally (cephalic flexure). By its 

 increase in size the brain has influenced the skull in an interesting way ; 

 for, while even in birds the brain is almost entirely confined to the region 

 behind the eyes, in the higher mammals it has extended forward to the 

 olfactory region. Thus there comes an increase of the cranium at the ex- 

 pense of the face. The relative sizes of the two were adopted by Camper 

 as an index of intelligence, and were measured by l Camper's angle,' a 

 method which has since undergone considerable improvements. 



Of the sense organs the nose is characterized by three features. 

 An outer nose, supported by cartilage and often extended as a 

 proboscis, has been formed. Its cavity has been increased, since 

 by the formation of hard and soft palate a part of the primitive 

 mouth cavity has been included in it. Its upper portion, the 

 olfactory region, has been complicated by the formation of olfactory 

 folds, supported by the turbinated bones already referred to (p. 

 620). To increase the mucous surface there are extensions of the 

 nasal cavity, frontal, maxillary, and sphenoidal sinuses, into the 

 corresponding bones. The eye has the upper and lower lids, besides 

 the nictitating membrane in a more or less reduced condition. 

 The ear, except in monotremes, Cetacea, Sirenia, and some seals, 

 has a conch supported by cartilage, while the external auditory 

 meatus is always present. Internally the ear is much modified, 

 since the three bones, malleus, incus, and stapes (p. 544), occur 

 nowhere else, while the lagena has been greatly lengthened, coiled 

 into a spiral with two to four turns (figs. 80, 576), while inside 

 the wonderful organ of Corti has been developed. 



Of digestive structures, the teeth which are restricted to max- 

 illary, premaxillary, and dentary bones need special mention, 

 because of the distinctions they afford from all other vertebrates, 

 and because of their importance in differentiating the various 

 orders. If we omit the monotremes, edentates, and whales, in 

 which there is marked degeneration in the dentition, there are 

 four particulars which show the dentition of mammals more de- 

 veloped than that of other vertebrates. (1) The number of teeth 



