THE MOUSE 433 



gland. Along the under side of the body, best developed in 

 the female, are two rows of milk glands. Each milk gland 

 is an aggregation of sebaceous glands whose secretion is modi- 

 fied to furnish a nutritive fluid for the young mammal. 



Two pairs of appendages are present, the hinder pair func- 

 tioning the more vigorously in locomotion. The entire hand or 

 foot does not lie flat on the ground as the sole of the human foot 

 does, but the wrist and heel are slightly elevated, while in the 

 horse the wrist and heel are elevated far from the ground, since 

 it stands on the tip of a single finger. In the rabbit the hand 

 has five digits, but the foot shows a reduction to four. 



Skeleton. — The vertebral colmnn of the rabbit, as of other 

 mammals, is divisiljle into five regions: (1) The neck, or 

 cervical vertebrae ; (2) the chest, or thoracic vertebriB ; (3) the 

 waist, or lumbar vertebrae ; (4) the pelvic, or sacral vertebrae ; 

 (.5) the tail, or caudal vertebrae. Of the neck vertebrae there 

 are seven, as in all other mammals whether the neck is short, 

 as in the whale, or long, as in the giraffe ; but the number of 

 the other vertebrae is variable. Each vertebra bears a spine 

 and processes for the attachment of muscles and ligaments. 

 The thoracic vertebrae bear ribs. The anterior ribs abut 

 ventrally upon a long rod, the breastbone, or sternum. The 

 skull is heavier than that of the bird ; the huge bone for the 

 lower jaw — the cjuadrate — is a part of the brain case or 

 cranium. A separate cheek-bone protects the eye cavity 

 below. 



The shoulder girdle consists on each side of two bones, forming 

 a V, which embraces between its arms half of the thorax, and 

 the skeleton of the appendages is fastened to the apex of the V. 

 The dorsal arm of the V is the shoulder-blade (scapula) ; this 

 is broad, and has a high ridge for the attachment of the muscles 

 2p 



