286 COMPARATIVE ANATOMY 



derives its name, are situated upon elevations of the skin 

 known as teats. The number and position of the teats differ 

 very much in different orders of mammals, their number 

 bearing a pretty constant relation to the number of young 

 produced at a birth. 



Typically the teats are numerous, and arranged in pairs 

 along the ventral surface of the thorax and abdomen, and in 

 those cases where few are present, the reduction takes place 

 at one or the other end of the series. In the rabbit there are 

 five pairs of teats, the first pair opposite the elbow, the last 

 pair in the inguinal region, between the thighs. In man and 

 apes only one pair of thoracic mammae are present, in sheep 

 and oxen only the posterior pairs of inguinal teats are deve- 

 loped. The mammary glands are formed, like the sebaceous 

 and sweat glands, as ingrowths of the Malpighian layer, which 

 penetrate the corium, acquire a lumen, and open to the 

 exterior. There are good grounds for regarding the mammary 

 glands as specially modified skin glands, like those already 

 mentioned. The tubules of the mammary glands are branched 

 and differentiated into terminal secretory lobules or acini, 

 ducts, milk reservoirs, etc. In the male, the teats and 

 mammary glands are rudimentary and functionless. 



The newly-born offspring would be unable to suck milk if 

 they were not furnished with freely movable lips ; this feature 

 is preserved in adult life, and is of great advantage in many 

 species, as anyone will realise who watches a herbivorous 

 animal cropping grass. 



The external nares and eyes of mammals occupy the same 

 relative positions as in lower vertebrates, and the eyes have 

 the same essential structure . and the same arrangement of 

 eye-muscles. The ears offer characteristic features. The 

 tympanum or drum of the ear is not situated on the surface, 

 as in the frog, but is placed at the bottom of a tube-like 

 depression, the external auditory meatus, and the entrance 

 to the meatus is guarded by a more or less considerable fold 

 of tissue, the external ear or pinna. The external ears of the 

 rabbit are of remarkable size^ and, as is the case in most 

 mammals, are freely movable by means of special muscles. 

 In man the external ears are reduced, and have lost their 

 mobility, but in a few individual cases a small amount of 

 movement is possible. 



