THE MAMMALIA. 



73 



bone. T!.e representative of the quadrate bone of the lower 

 Vertebrata is appropriated, as malleus, to the service of the organs 

 of hearing. 



The brain possesses a corpus callosum. 



The greater and lesser circulations of Mammals are as com- 

 pletely distinct as in Birds, and there is but a single aortic arch, 

 the left. The majority of the blood-corpuscles are red, free 

 nuclei, and these are always discoidal, and usually circular in 

 form. The blood is hot. There is a complete diaphragm, and 

 none of the bronchi end in air-sacs, like those of Birds. 



Fig. 38. 



FT. 



No, 



P-7713-. 



Fig. 38. The skull of a Dog. D. Ramus of the lower jaw ; Sq. Squamo&il. 



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 papillte 

 inclosed within sacs; but, on the other hand, differ from the 

 horny appendages of birds, in not splitting up as they are pro- 

 truded, in the fashion so characteristic of feathers. 



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 



