GENERAL ZOOLOGY 



Fig. 3.10. Electron micrograph of a section through lung of rat; x26,000. (From the 

 original of Fig. 5, Frank N. Lx)w, Anatomical Record, vol. 117, p. 257, printed by per- 

 mission of the author and publishers.) 



The larynx, trachea, and bronchi are prevented from collapsing by cartilag- 

 inous supports in their walls (p. 61). 



Lungs in lower vertebrates such as the frog are sac-like, with the inner 

 respiratory surface area increased by a series of folds of the lining membrane. 

 In higher vertebrates, such as man, the bronchi branch repeatedly in the 

 lungs forming bronchioles,- in the smaller bronchioles cartilaginous rings are 

 no longer present. These air passages end in small expansions known as the 

 alveoli or air chambers (Fig. 3.9). Each alveolus is lined by a single layer of 

 cells and, as in the gill, this rests on a rich capillary bed (Fig. 3.10). 



In the frog the lungs lie in the pleuroperitoneal portion of the coelom. In 

 the course of evolution in the vertebrates, this cavity becomes partitioned by a 

 muscular structure known, in mammals, as the diaphragm (Fig. 3.2). Now, 

 each lung lies in a closed pleural cavity, one on each side of the heart, 

 surrounded by the thoracic cage (p. 92). Breathing is the result of in- 

 creasing and decreasing the volume of the pleural cavities by movements of 

 the diaphragm and ribs. The control of this mechanism will be discussed 

 later (p. 124). 



The Excretory System. The lungs, the skin, the liver, and the excre- 

 tory organs are organs in which excretion occurs, but they do not make up a 

 system of organs in the ordinary meaning of the phrase. In dividing the 



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