552 CHORDATA 



transverse commissure, the pons Varolii, passes l.ielow, connecting the 

 two sides of tlie cercliellum. Tl\ese connexions in the other vertebrates 

 are small, and e\en in the lower mammals, like monotremes and marsu- 

 pials, are but sUghtly dc\eloped. 



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

 dorsal poruon, 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 vcntrally in the medullary region (cervical flexure), 

 then in the region of the pons again dorsally (poiital flexure'), and at the lc\-el of 

 the optic lobes again \entrally {cephalic flexure). By its increase in size the 

 brain has influenced the skull in an interesting way; for, while e\en in Ijirds the 

 brain is almost entirelv confined to the region behind the eyes, in the hit^her 

 mammals it has extended forward to the olfactory rcLjion. 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 extendetl as a proboscis, 

 has been formed. Its cavity has been increased, since by the formation 

 of hard and soft palate a part of tlie primitive mouth cavity has l)cen 

 included in it. Its upper portion, the olfactory region, has lieen compli- 

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

 already referred to (p. 54S). The eye has the upper and lower lids, 

 besides the nictitating meml.irane in a more or less reduced condition. 

 The ear, except in monotremes, cetacea, sirenia, and some seals, has a 

 conch supported liv cartilage, while the external auditory meatus is alwavs 

 present. Internally the ear is much modi lied, since the three bones, 

 malleus, incus, and stapes (p. 479), occur nowhere else, while the lagena 

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

 (tigs. S3, 533), insitle of which is the wonderfid orgaji of Corli. 



Of digestive structures, the teeth, restricted to maxillary, premaxillarv, 

 and dentary bones, need special mention, because of the distinctions thcv 

 afford from all other vertel.irates, and because of their importance in 

 differentiating the various orders. If we omit the monotremes, edeir- 

 tates, and whales, in \\hich there is markeil degeneration in the dentition, 

 there are four particulars which show the dentition of mammals more 

 developed than that of other vertebrates. (1) 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 plat)'rhine apes thirty-six, etc. (2) The 

 teeth are more hrmly lixed. The body of dcutiitc is divided, by a slight 

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

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



