IV THE ORGANS OF RESPIRATION 129 



on the windpipe of a living animal so as to prevent air 

 from passing into, or out of, the lungs, and then examin- 

 ing the contents of the heart and great vessels. The 

 blood on both sides of the heart, and in the pulmonary 

 veins and aorta, will then be found to be as completely 

 venous as in the venae cav?e and pulmonary artery. 



But though the passage of carbonic acid (and hot 

 watery vapour) out of the blood and of oxygen into it 

 is the essence of the respiratory process — and thus a 

 membrane with blood on one side, and air on the other, 

 is all that is absolutely necessary to effect the purification 

 of the blood — yet the accumulation of carbonic acid is so 

 rapid, and the need for oxygen so incessant, in all parts 

 of the human body, that the former could not be cleared 

 away, nor the latter supplied,, with adequate rapidity, 

 without the aid of extensive and complicated accessory 

 machinery — the arrangement and working of which must 

 next be carefully studied. 



3. The Organs of Respiration.— The back of the 

 mouth or pliar3nix communicates by two channels with 

 the external air (see Fig. 37, gf./.e.). One of these is 

 formed by the nasal passages, which cannot be closed by 

 any muscular apparatus of their oa\ti ; the other is 

 presented by the mouth, which can be shut or opened at 

 will. 



Immediately behind the tongue, at the lower and front 

 part of the pharynx, is an aperture — the glottis (Fig. 

 38, Gl) — capable of being closed by a sort of lid — the 

 epiglottis (Fig. 37, e.) — or by the shutting together of 

 its side boundaries, formed by the so-called vocalcords. 

 The glottis opens into a chamber with cartilaginous walls 

 — the larynx ; and leading from the lai-jTix do-\\-nwards 

 along the front part of the throat, where it may be very 

 readily felt, is the trachea, or windpipe (Fig. 37, c, Fig. 

 38, Tr.). 



If the trachea be handled through the skin, it will be 

 found to be firm and resisting. Its waUs are, in fact, 



E 



