72 



ON CLASSIFICATION. 



Some part or other of the integument of all Mammals exhibits 

 "hairs" — horny modifications of the epidermis — which so far 

 resemble feathers, that they are developed upon papillae inclosed 

 within sacs ; but, on the other hand, differ from the horny ap- 

 pendages of birds, in not splitting up as they are protruded, in 

 the fashion so characteristic of feathers. 



Fig. 36. 



Kijr, 36. — The skull of a Dog. — D. Ramus of the lower jaw; Sq. Squamosal. 



Finally, all Mammals are provided with organs for the secre- 

 tion of a fluid which subserves the nourishment of the young 

 after birth. The fluid is milk ; the organs are the so-called 

 "mammary" glands, and may probably be regarded as an ex- 

 treme modification of the cutaneous sebaceous glands. These 

 glands are aggregated into two or more masses, disposed upon each 

 side of the median line of the ventral surface of the body ; and, 

 in almost all Mammals, the aggregated ducts of each mass open 

 upon an elevation of the skin common to all — the nipple or 

 teat. To this the mouth of the newly-born Mammal is applied, 

 and from it, either by suction on the part of the young, or by 

 the compressive action of a special muscle on the part of the 

 parent, the nutritive fluid makes its way into the stomach of the 

 former. 



