446 ZOOLOGY FOR MEDICAL STUDENTS chap. 



is that the lining of the nose grows out to form complicated recesses 

 between which remain thin partitions supported by bone and covered 

 with sensory epithelium underlaid by a rich network of blood-vessels to 

 warm the inspired air. In the dried skull the thin layers of bone support- 

 ing these partitions are seen as more or less complicated scrolls and 

 spongeworks of bone known as turbinals — ^almost blocking up the nasal 

 cavities in many of the mammals which breathe particularly cold air, 

 e.g. polar bears and seals. 



It is also characteristic of the mammal that the communication 

 between the nose and the mouth becomes shunted backwards towards 

 the glottis. This is brought about by the dorsal portion of the buccal 

 cavity, into which open the posterior nares, becoming separated off by 

 a floor — the " hard palate " — formed by the meeting of shelf-like bony 

 ingrowths from the premaxillae, maxillae and palatine bones. It is of 

 interest that a precisely similar displacement backwards of the com- 

 munication between nose and mouth has taken place in the crocodiles 

 amongst reptiles, though here it has been carried to a still further extent 

 than in mammals. 



In particular cases the limb skeleton of the mammal is greatly 

 modified in correlation with the specialization of the limb for particular 

 modes of progression : as in the simplified flipper of the whale, the wing 

 of the bat with its immensely elongated slender digits for the support 

 of the thin wing-membrane, or the foot of the horse with its solitary 

 remaining toe — but on the whole the limb skeleton is characterized by 

 a much lower degree of specialization than that of the bird. It has as 

 a rule not departed very widely from the general plan of pentadactyle ' 

 limb illustrated on page 404. It is characteristic of the typical mammals 

 that the coracoid portion of the shoulder girdle has become reduced to 

 an insignificant vestige, the " coracoid process,'' which projects from the 

 scapula. In those mammals in which it is necessary the function of 

 the coracoid to prevent the scapula from being drawn down towards 

 the sternum by muscular contraction is carried out by the clavicle or 

 collar-bone. 



The Brain of the mammal exhibits the same main regions as that of 

 other vertebrates J There are various differences in detail. In correlation 

 with the complicated co-ordinations of muscular contraction involved in 

 the movements and balancing of the body the cerebellum is highly de- 

 veloped. Its superficial layers grow actively in area and become in 

 consequence richly folded : its lateral portions are much enlarged (cere- 

 bellar hemispheres) and are connected together by a large ventrally 



