464 



CHORD AT A. 



Blood -Vascular System. — The heart and circulatory 

 system do not show any great adaptation throughout the 

 class. The heart is always four chambered and the systemic 

 arch is only found on the left. In various regions of the 

 body there are developed fine meshworks of blood-vessels 

 termed retia mirabilia. These are found at the bases of 

 the limbs in many arboreal animals which have to hang 

 from boughs, in which case they appear to counteract the 



Fig. 322. — A Rete Mirabile. 



A, General appearance. B, Cross-section of the blood-vessels. C, Anastomosis of 

 smaller and larger vessels. (After Murie.) 



retarding effects of gravity upon the circulation (cf. Sloth). 

 They also occur in whales, possibly for the storage of arterial 

 blood to allow of a long sojourn under water. 



Respiratory System. — The lungs lie freely in the 

 thoracic cavity, being completely surrounded by the pleura, 

 and respiration is effected by the ribs and intercostal 

 muscles, supplemented by the diaphragm, as in the rabbit. 

 The diaphragm is foreshadowed in types like the crocodile, 

 but it is typically a mammalian organ in its perfect condition. 



