CHORDATA — VERTEBRATA 333 



of the proteids; the inorganic salts and much water are likewise 

 brought to the kidneys. From the heart it is all pumped to the 

 lungs, where much of the carbon dioxid is given up by the blood 

 and oxygen is taken into it. 



Body waste. — Thus the solid waste passes out of the body 

 by way of the digestive canal ; the liquid waste by means of 

 the kidneys through the bladder, and also through the agency 

 of the sweat glands ; the gaseous, through the lungs. 



Respiration is performed by the lungs, — a hollow, blind, 

 spongy outpushing from a part of the throat (pharynx). Air 

 is drawn into and forced out of these, through nasal passages, 

 pharynx, larynx, and trachea, by the contraction and relaxa- 

 tion of the muscles of the chest. Since a network of blood cap- 

 illaries lines each of the myriad of minute air-chambers, the 

 interchange of carbon dioxid and oxygen is rapid. The oxygen 

 carrier of the blood is the haemoglobin of the red blood corpuscles, 

 for which oxygen has a strong affinity. 



Voice. — An outgrowth of this method of respiration is the 

 development of voice. As the air passes to or from the lungs it 

 must traverse the larynx (Fig. 142), upon the sides of which are 

 developed folds, — the vocal cords ; the rapid passage of the air 

 over these produces an audible vibration. As muscles are at- 

 tached to the ends of these, they can be expanded and con- 

 tracted, thus giving rise to variations in the pitch of the voice. 



Nervous systems. — The nervous elements form (i) the 

 central nervous system, and (2) the peripheral nervous system. 

 The former consists of the brain and spinal cord; the latter 

 is made up of those nerves which establish a connection be- 

 tween the periphery of the body and this central system. The 

 brain, situated in the posterior portion of the skull, gives o^, 

 especially to the muscles of the head and neck, twelve pairs of 

 cranial nerves. The spinal cord, which is continuous with the 

 brain and occupies a tube extending through the spinal column, 

 gives off especially to the muscles of the legs and trunk, forty 

 pairs of spinal nerves. The cranial and spinal nerves with the 



