328 ELEMENTS OF COMPARATIVE ZOOLOGY. 



ceases, for in the vertebrates the air is brought from the 

 lungs to the working parts by the intervention of the 

 nutrient fluid (blood). 



The methods by which air is drawn into the lungs vary. 

 The frog swallows the air by aid of the muscles extending 

 across the throat between the halves of the lower jaw, and 

 that this swallowing is the only way of forcing air into the 

 lungs is shown by the fact that if the mouth be kept from 

 closing the animal will suffocate.* In the Sauropsida the 

 muscles between the ribs and those forming the walls of the 

 abdomen are concerned in the inspiration and expiration of 

 air; while in mammals the muscular partition (diaphragm) 

 which divides the body-cavity becomes an efficient organ 

 in the process. 



We naturally think of work in terms of motion, and in 

 the case of an animal the contraction of a muscle or the 

 movement of a part or the whole of the body naturally sug- 

 gest themselves as examples. These, however, are but a 

 part of the work which the animal does. The performance 

 of any function of the body is really work. When a gland 

 secretes, a nerve acts, an intestine absorbs, or the mind 

 carries on its operations, the expenditure of energy is called 

 for just as in the contraction of a muscle. So all parts 

 must have both food and oxygen. 



When coal is burned in an engine, besides energy there 

 is a production of waste. A part of this waste passes off in 

 a gaseous condition as water vapor and part as ashes. 

 When any part of the animal body works there is a similar 

 formation of waste, and the carbon dioxide and water vapor 

 are carried away by the same structures (tracheae in the 

 insects, blood-vessels and gills or lungs in many other forms) 

 which brought the oxygen to the parts. 



* The skin is a very important organ in the respiration of the 

 Batrachia (see p. 50). 



