MAMMALS 



17 



other animals. Extending from it is the long, protected 

 cerebro-spinal cord, with its myriads of nerve branches. 

 All the impulses arise in the brain, and what are equiva- 

 lent to orders are sent along the nerves to the various 

 organs, which obey the desire or impulse. 



Fig. 13. — Distribution of Air 

 Tubes in Mammalian Lungs. 



a, larynx ; b, trachea ; c, d, left and right 

 bronchial tubes ; £,/,£■, the ramifi- 

 cations. In man the subdivision 

 continues until the ultimate tubes 

 are one twenty-fifth of an inch in 

 diameter. Each lobule represents 

 in miniature the structure of the 

 entire lung of a frog. 



Fig. 12. — Venous Valves. 



They usually occur in pairs, as repre- 

 sented. 



All the mammals, if we 

 except the mole, have well- 

 developed eyes, and all but the whale and some seals have 

 external ears. Nearly all mammals are covered with hair, 

 the whale being an exception. Some, as the armadillos, 

 are protected by horny plates. 



holder, mammals — 2 



