778 ANATOMY OF VERTEBRATES. 



sucking must be performed in the present aquatic mammals : 

 f As cither the mother or young one will be prevented from 

 breathing at the time, their nostrils being in opposite directions, 

 therefore the nose of one must be under water, and the time of 

 sucking can only be between each respiration.' The considerable 

 lacteal reservoir, and the quality of milk it contains, 1 relate to this 

 difficulty. 



Some stress has been laid on the assistance which the muscles 

 in contact with the mammary gland might afford by compressing 

 the gland and ejecting the milk accumulated in the dilated recep- 

 tacle ; ' but,' as I remarked in CXLIV". p. 594, ' when we consider 

 how great the pressure of the surrounding water must be upon 

 the extended surface of the mammary gland, we may readily con- 

 ceive that when the nipple is grasped by the mouth of the young, 

 and the pressure removed from it by the retraction of the tongue, 

 the milk will be expelled in a copious stream by means of the 

 surrounding pressure alone, independently of muscular aid.' 



In Sirenia the mammary glands, also a pair, are pectoral in 

 position ; the teats are prominent and conspicuous at the suckling 

 season. The mother has been seen holding her young to the 

 breast, with one flipper, and maintaining both her own and her 

 offspring's nostrils above water. The resemblance to a black 

 woman and child has attracted attention, and the appearance of 

 the tail-fin as she dived, has served to perpetuate the seaman's 

 faith in the ' mermaid.' 



417. In Ungulata,- -The Elephant resembles the Dugong in 

 the number and position of the mammary glands and teats, which 

 project between the fore-legs. The young animal compresses the 

 gland with its proboscis as its sucks. 



In the unimpregnated Rhinoceros the mammary glands, two in 

 number, form a thin layer expanding forwards beneath the ab- 

 dominal integument, between the dermal and abdominal muscles. 

 The nipples are two in number and inguinal, are situated 14 

 inches in advance of the vulva, and 2J inches apart from one 

 another. They are subcompressed, obtusely rounded at the 

 extremity, and about 2 inches in length : about a dozen lacti- 

 ferous ducts open upon the someAvhat flattened summit of each 

 nipple. 



In the Mare and Ass the mammary glands, two in number, are 

 situated between the thighs at a distance of about 9 inches in 



1 ' The milk is probably very rich ; for in that caught near Berkeley with its young 

 one, the milk, which was tasted by Mr. Jenner, and Mr. Ludlow, surgeon, at Sodbury, 

 was rich like cow's milk to which cream had been added.' xciv. p. 392. 



