422 COMPAEATIVE ANATOMY. 



of glands at its base which, grow into the corium; this area is 

 marked off from the surrounding integument by a circular elevation 

 (a,). This arrangemeut corresponds to the mammary pouch in 

 Echidna. In most of the Mammalia this arrangement is not a per- 

 manent one ; it soon becomes level again, and the glandular area has 

 then its central part (5), in which the glands open, raised up into a 

 papilla or nipple; at the tip of which a number of gland-ducts 

 always open. 



In the other arrangement the mammary pouch is persistent. 

 Owing to the continued elevation of the peripheiy of the gland («) the 

 glandular area is more and more depressed, the edge of the mammary 

 pouch is developed into a pseudo-nipple, from the tip of which a 

 single canal passes to the glandular area (G). 



This ari'angement has been observed in some of the Ungulata. 

 Intermediate stages between the two arrangements can be made out 

 in the Marsupialia (Halmaturus) andRodentia (Murina). The number 

 of mammary glands which are distinguished by their nipples 

 varies in different divisions. They generally correspond to the 

 average numbei', or to the maximum number of young produced at 

 one birth. They vai'y even within the limits of the same order ; and 

 they also vary in position. As a rule they form two rows, which, 

 when there is a large number present, extend from the inguinal to 

 the pectoral region (Carnivora, Suina). In many of the Didelphia 

 they are arranged in a circle on the abdomen. When the number 

 is not so large, they either occupy an abdominal position, as in many 

 Didelphia, or they are only found in the lumbar region (Perisso- 

 dactyla^ Euminantia, Cetacea), or, finally, they are limited to the 

 pectoral region (Elephant, Sirenia, many Prosimii, Chiroptera, and 

 Primates). When more than one pair is present some glands are 

 sometimes aborted, so that there are rudimentary organs present, 

 together with well-developed and functionally active ones; and 

 these may be recognised by their rudimentary nipples. In a 

 similar way the whole apparatus is atrophied in the male. 



The most important adaptation of the integument to the function 

 performed by the mammary glands, is the formation of the folds 

 of integument found in the Marsupialia; these form a sac, the 

 marsupium, which encloses the mammiferous region of the ab- 

 domen. The extent of its development appears to vary inversely 

 with the extent to which the young are developed at birth. 



Dermal Skeleton. 



§ 324. 



The function of the integument as a defensive organ for the 

 body is increased in value by the formation of hard structures. 

 When these parts are of some size they give rise to a dermal 



