ZOOLOGl . 



in tin- gurroanding water; whilst in the more highly organized, 



tin- mollusks mid fishes, the i>ranchi;e or 

 i,nlls are enclosed in a cavity, into which the 

 water has free BOOM*, and may easily be re- 



lleweil. 



133. Organs of Aerian Rcx^'iral'm,, . 

 Hi-.*],;, -at inn in Air. The organs serving tor 

 this form of respiration aU'ect sometimes the 

 |MI in of trachea?, sometimes of lungs. 



The tracheae (Fiir- l s > ' v vessels which 

 communicate with the exterior by apertures 

 called xtiijnidhi. and ramify in the depth of 

 the organs. They convey the air to these 

 organs, and thus the function of respiration 

 is carried on in every part of the body. This 

 mode of breathing is peculiar to inserts and to 

 some arachnid (spiders). 



134. The lungs are pouches, more or less 

 divided into cells or cellules, which also re- 

 ceive air into their interior, and whose walls are 

 traversed by vessels containing blood, thus 

 exposed to the vivifying action of the air. 



There exist lungs, but in a state of the 

 greatest simplicity, in most araignees (spiders); 

 and in some mollusks, as in snails. Reptiles, 

 birds, and mammals also have lungs. 



135. In man, as in all mammals, the 

 lungs are lodged in a cavity, called the thorax, 

 occupying the upper part of the trunk (Fig. 4, 

 p. 20). These organs are, as it were, suspended 

 in this cavity, and are enveloped by a mem- 

 brane or membranes (one for each lung in 

 most mammals) called the pleura?; and these 

 membranes, like all serous membranes, be- 

 sides investing more or less completely the 

 exterior of the organs, also line the walls 

 of the thorax, thus providing for the security 

 i organs tm 



and mobility of the organs themselves.* The 



ire two in number, communicating 

 with the exterior by means of a single air- 

 tube. the trachea (/;. Fig. 19), which ascends 

 through the fore part of the neck, and opens into the pharynx. 



Fip. 47 The 

 \fiiicola. 



The pleurae are arranged, in fact, like other serous membranes. 



