568 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



Fig. 243. — Transverse Sec- 

 tion OF A BHANCHIOPOD, 

 SHOWING THE LEAF-LIKE 



Gills, after Grube. (Jef- 

 frey Bell.) 

 C, heart; I, intestine; N, ventral 



nerve-cord ; D, fold of the integument ; 



BR BR', gills, which are appendages of 



the hody. 



Vertebrates are the only animals that breathe through the nostrils or 

 mouth. Fishes inspire only, and all vertebrate animals in whom the ribs 



are absent or solidly fastened together 

 swallow air. 



In reptiles we first meet with a complete 

 adaptation of a pulmonary structure for the 

 direct aeration of the blood through the 

 influence of the atmosphere. In them there 

 is an internal prolongation of the external 

 integument, constituting the lungs, though 

 they exhibit great simplicity, being, for the 

 most part, capacious sacs, occupying con- 

 siderable bulk, but being but slightly sub- 

 divided, so that the amount of surface 

 exposed is really very small, the blood being 

 exposed on one surface only to aeration. 

 The greatest diversity is met with in these animals as regards the 

 structure and mobility of the thorax. In the saurians, the thorax 

 resembles that of mammals, with movable ribs ; in the chelonia, the 



thoracic walls are rigid and immovable ; in the 

 ophidians, the ribs are very numerous and 

 movable, the sternum being absent. A dia- 

 phragm is met with only in the higher sau- 

 rians. In reptiles inspiration is not accom- 

 plished by inhalation, but by deglutition, air 

 being drawn into the pharynx by depression 

 of the hyoid apparatus, and the nares then 

 being closed, the air is forced into the trachea. 

 Expiration is accomplished mainly by the 

 elasticity of the lungs, aided by the abdom- 

 inal muscles, and in saurians and ophidians 

 by the intercostal muscles and the elasticity 

 of the chest-walls. In snakes, as a rule, 

 there is a single, long, cylindrical lung, 

 while the left lung is rudimentary. 



In birds, though the diaphragm is still 

 absent or rudimentary, the respiratory appa- 

 ratus is more complicated than any yet 

 considered, so that the energy of the respira- 

 tor process is much increased, and yet 

 the general plan of the apparatus is much more closely allied to that of 

 reptiles than of mammals. For each lung may be considered to be sub- 

 divided into lobules, each of which resembles the rudimentary lung of 



Fig. 244.— Gill of the Perch. 

 (Jeffrey Bell.) 

 A, hranchial artery : B, branchial arch 

 (seen in cross-section) ; C, branches of the 

 branchial vein, V ; D, branches of the 

 branchial artery. 



