116 RESPIRATION. 



In treating in detail of the function of respiration, it will be convenient to make the 

 following division of the subject: 



1. The mechanical phenomena of respiration ; or the processes by which the fresh air 

 is introduced into the lungs (inspiration), and the vitiated air is expelled (expiration). 



2. The changes which the air undergoes in respiration. 



3. The changes which the blood undergoes in respiration. 



4. The relations of the consumption of oxygen and the production of carbonic acid to 

 the general process of nutrition. 



5. The respiratory sense ; a want, on the part of the system, which induces the re- 

 spiratory acts (besoin de respirer). 



6. Cutaneous respiration. 



7. Asphyxia. 



The study of these questions will be facilitated by a brief consideration of some 

 points in the anatomy of the respiratory organs. 



Physiological Anatomy of the Respiratory Organs. 



Passing backward from the mouth to the pharynx, two openings are observed; 

 a posterior opening, which leads to the oesophagus, and an anterior opening, the opening 

 of the larynx, which is the commencement of the passages devoted exclusively to respi- 

 ration. The structure of the oesophagus and of the air-tubes is entirely different. The 

 oesophagus is flaccid and destined to receive and convey to the stomach the articles of 

 food, which are introduced by the constrictions of the muscles above. The trachea and 

 its ramifications are exclusively for the passage of air, which is taken in by a suction 

 force produced by the enlargement of the thorax. The act of inhalation requires that 

 the tubes should be kept open by walls sufficiently rigid to resist the external pressure 

 of the air. 



Beginning our description with the larynx, it is seen that the cartilages of which it is 

 composed are sufficiently rigid and unyielding to resist the pressure produced by any in- 

 spiratory effort. Across its superior opening are the vocal chords, which are four in num- 

 ber and have a direction from before backward. The two superior are called the false vocal 

 chords, because they are not concerned in the production of the voice. The two inferior 

 are the true vocal chords. They are ligainentous bands covered by folds of mucous mem 

 brane, which is quite thick on the superior chords and very thin and delicate on the in- 

 ferior. Anteriorly, they are attached to a fixed point between the thyroid cartilages, and 

 posteriorly, to the movable arytenoid cartilages. Air is admitted to the trachea through 

 an opening between the chords, which is called the rima glottidis. Little muscles, arising 

 from the thyroid and cricoid and attached to the arytenoid cartilages, are capable of 

 separating and approximating the points to which the vocal chords are attached posteri- 

 orly, so as to open and close the rima glottidis. 



If the glottis be exposed in a living animal, certain regular movements are presented, 

 which are synchronous with the acts of respiration. The larynx is opened at each inspira- 

 tion by the action of the muscles referred to above, so that the air has a free entrance to 

 the trachea. At the termination of the inspiratory act, these muscles are relaxed, the 

 vocal chords fall together by their own elasticity, and, in expiration, the chink of the 

 glottis returns to the condition of a narrow slit. These respiratory movements of the 

 glottis are constant and are essential to the introduction of air in proper quantity into the 

 lungs. The expulsion of air from the lungs is rather a passive process and tends in it- 

 self to separate the vocal chords ; but inspiration, which is active and more violent, were 

 it not for the movements of the glottis, would have a tendency to draw the vocal chords 

 together. The muscles which are concerned in producing these movements are animated 

 by the inferior laryngeal branches of the pneumogastric nerves. If these nerves be 

 divided, the movements of the glottis are arrested, and respiration is very seriously inter- 



