THE RKSPIRATOIiy SYSTEM. 



371 



the vertical diameter also. All these events, favorable to 

 the entrance of air, actually take place through agencies we 

 must now consider. The student is recommended to look into 

 the insertion, etc., of the muscles concerned, to which we can 

 only briefly refer. We have made the descriptions and cuts 

 applicable to man, so that it may be easy for the student to ver- 

 ify all essential points on his own person. Eespiration in our 

 domestic animals is in the main as in man. 



The act of inspiration commences by the fixation of the 

 uppermost ribs, beginning with the first two, by means of the 



Fig. 393. 



Fig. a94. 



Fig. 293. — ^Diagram illustrating elevation of ribB in inspiration (B€clard). The dark 

 lines represent the ribs, sternum, and costal cartilages in inspiration. 



Fig. 294.— Diagrammatic representation of action of diaphragm in respiration (Her- 

 mann), vertical section throngh second rib on right side. The broken and dot- 

 ted lines show the amonnt of the descent of the diaphragm in ordinary and in 

 deep inspiration. 



scaleni muscles, this act being followed up by the contraction of 

 the external intercostals, leading to the elevation of the other 

 ribs ; at the same time, the arch of the diaphragm descends in 

 consequence of the contraction of its various muscular bundles. 

 Under these circumstances, the air from without must rush in, 

 or a vacuum be formed in the thoracic cavity ; and, since there 



