26 ZOOLOGY. 



CHAPTEB, II. 



VERTEBRATA. . 

 CLASS I, Mammalia. 



22. The highest class of vertebrate animals is termed 

 Mammalia (Lat. mamma, the breast). It is so called 

 because the young, being brought forth in a state more or 

 less helpless, are, for a time, nourished by a fluid called 

 milk, secreted by a set of glands termed "mammary 

 glands." 



The following are the most important characteristics 

 of the Mammalia : 



They are covered with hair; the skull is united to the 

 vertebral column by two occipital condyles; each half 

 of the lower jaw consists of a single piece; the lower jaw 

 is joined directly with the skull, and not to the quad- 

 rate bone; the heart has four cavities two auricles and 

 two ventricles; the red blood-corpuscles are circular, and 

 non-nucleated; the blood is hot; respiration is always 

 effected by lungs; the diaphragm completely separates 

 the cavities of the thorax and abdomen; the hemispheres 

 of the brain are united by a corpus callosum ; the embryo 

 is provided with an amnion and an allantois ; the young 

 are nourished by milk, secreted by mammary glands. 



23. Skeleton. The skull in the Mammalia is united to 

 the vertebral column by two condyles on the occipital 

 bone. There are two corresponding "facets" on the 

 atlas, or upper joint of the vertebral column. Each half 

 of the lower jaw consists of only a single piece, and is 

 articulated directly with the skull, and not to the quad- 

 rate bone, as in birds and reptiles. 



24. The divisions of the vertebral column usually 

 recognized are : 



1 . The cervical (Lat. cervix, the neck) region or neck. 



