THE MAMMALIA 293 



has a remarkably reptilian appearance, as the coracoids are 

 well developed, and extend to and articulate with the sternum. 



The chief characteristic of the pelvic girdle of mammals is the 

 fact that its long axis is nearly parallel to the vertebral column. 

 The ilia are generally large and expanded, and usually the 

 pubes, less frequently the ischia, meet in ventral symphyses. 



The fore and hind limbs are formed on the pentadactyle 

 type, which has already been fully described for the frog 

 (vol. i. p. 35). The fourth and fifth carpals and the 

 corresponding tarsals in mammals are commonly fused to 

 form the unciform and cuboid bones respectively. The 

 digits both of the manus and pes are commonly reduced 

 in number, this reduction being most conspicuous in the 

 horse, whose hoof is the modified nail or claw of the ungual 

 phalanx of the third digit, the second and fourth digits being 

 reduced to rudimentary spHnt-bones, and the first and fifth 

 altogether absent. In the rabbit the hand has five fingers, 

 of which the first, or pollex, is much shorter than the others, 

 while in the foot there are only four toes, the hallux or great 

 toe having disappeared. 



If we now examine the internal organs, we find that in 

 mammals the abdominal part of the coelom is completely 

 shut off from the thoracic portion by a muscular partition 

 called the diaphragm. The diaphragm consists of a central 

 tendinous portion, the centrum tendineum, from which flat 

 bands of muscle radiate in all directions, and are attached 

 to the vertebral column, the floating ribs and the xiphisternum. 

 When its muscles are relaxed, the diaphragm is arched up to- 

 wards the thoracic cavity ; when its muscles contract, it is 

 flattened and the thoracic cavity is enlarged. The oesophagus, 

 the inferior vena cava, and the dorsal aorta pass through 

 special holes in the diaphragm, but there is no communication 

 by way of these holes between the thoracic and abdominal 

 cavities. The oesophagus and blood-vessels are fastened by 

 connective tissue to the edges of the holes, and the thorax 

 is an air-tight box, enclosed by the ribs and diaphragm. The 

 thoracic cavity contains the heart and lungs, the bases of the 

 great arteries and veins, and a large glandular mass of doubtful 

 function, the thymus. The heart is enclosed in a double 

 membranous sac, the pericardium, and the lungs are similarly 

 enclosed in double membranous sacs called pleura. The 



Digitized by Microsoft® 



