552 CHORDATA 



transverse commissure, the pans Varolii, passes below, connecting the 

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

 are small, and even in the lower mammals, like monotremes and marsu- 

 pials, 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 indi- 

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

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

 the axis of the brain bends ventrally in the medullary region (cervical flexure), 

 then in the region of the pons again dorsally (pontal flexure), and 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 expense of the face. 



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 compli- 

 cated by the formation of olfactory folds, supported by the turbinal bones 

 already referred to (p. 548). 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. 479), occur nowhere else, while the lagena 

 has been greatly lengthened, coiled into a spiral with two to four turns 

 (figs. 83, 533), inside of which is the wonderful organ of Corti. 



Of digestive structures, the teeth, restricted to maxillary, 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, eden- 

 tates, and whales, in which there is marked degeneration in the dentition, 

 there are four particulars which show the dentition of mammals more 

 developed than that of other vertebrates, (i) The number of teeth is 

 constant for the species, usually for the genus, and often for the family. 

 As man normally has thirty-two teeth, so the dog has forty-two, the 

 anthropoid apes thirty-two, the platyrhine apes thirty-six, etc. (2) The 

 teeth are more firmly fixed. The body of dentine is divided, by a slight 

 constriction, into a crown covered with enamel, and a root enveloped in 

 cement (bony tissue). The roots are placed in separate sockets (alveoli) 



