374 



COMPARATIVE PHYSIOLOGY. 



wMcli are not normally active at all or" but very slightly in nat- 

 ural breathing. 



Facial and laryngeal respiration is best seen in such ani- 

 mals as the rabbit, and it is this condition which is ap- 

 proximated in disordered states in man — in fact, when from 

 any cause inspiration is very labored (asthma, diphtheria, 



etc.). 



In man and most mammals, unlike the frog, the glottic 

 opening is never entirely closed during any part of the respira- 

 tory act, though it undergoes a rhythmical change of size. 



Fig. ! 



Fig. 299. 



Fig. 298. — Vertical transverse section of fresh-water mussel (Anodon.) through heart 

 (after Huxley). V, ventricle; a, auricles; r, rectum; jo, pericardium; i. mner, o, 

 outer gill: o', vestibule of organ of Bojanus, B; /. foot; m, m, mantle lobes. 



Fig. 299.— Gill of flsh (perch), to illustrate relations of different blood-vessels, etc., 

 concerned in respiration (after Bell). A, branchial artery; B, branchial arch 

 seen in cross-section; F, branchial vein; «, v> branches of artery and vein re- 

 spectively. 



widening during inspiration and narrowing during expiration, 

 in accordance with the action of the muscles attached to the 

 arytenoid cartilages, the action of which may be studied in man 

 by means of the laryngscope. 



The abdominal muscles have a powerful rhythmical action 

 during forced respiration, though whether they function dur- 



