226 THE GAT. [CHAP. vm. 



The alternation of acts of inspiration and expiration (one of the 

 former commencing the series at birth, and one of the latter 

 terminating it at death) goes on unceasingly, but at a rate which 

 varies according to circumstances being most frequent during 

 violent exertion. 



What is called the respiratory rythm consists of three parts: 

 (1) the act of inspiration ; (2) that of expiration, which endures 

 hut little more than half as long as the former ; and (3) an interval 

 of rest, which is much shorter than either. 



Inspiration aids the circulation indirectly by pressing on the great 

 vessels, the difference between the strength of the walls of the 

 arteries and veins causing the pressure to be no impediment to the 

 former, while the same pressure aids the flow of the blood in the 

 latter. 



APPLICATIONS OF THE RESPIRATORY ACTIONS produce a number 

 of familiar actions, such as y aiming, which is a prolonged inspira- 

 tion ; coughing and sneezing, which are sudden acts of expiration, 

 the former being preceded by a prolonged inspiration the air 

 passing out by the mouth. In the latter it passes out only through 

 the nose. All forms of mewing, howling, and other vocal manifesta- 

 tions, are modified expiratory actions. 



The term VITAL CAPACITY refers to the capacity of the lungs as 

 estimated by the greatest quantity of air which can be expelled 

 from the lungs by the most forcible expiration after they have been 

 inflated by the deepest inspiration. But no expiration, however 

 violent or prolonged, will nearly expel the air which the lungs can 

 be made to contain while in ordinary respiration but a very small 

 part ebbs and flows. 



This small quantity is called the breathing or tidal air. That 

 which always remains and can never be expelled is called the 

 residual air ; that which ordinarily remains in the lungs after 

 expiration, but which can be expelled, is called the reserve or sup- 

 plemental air, and that which can be drawn in by a prolonged 

 inspiration, beyond that ordinarily so taken, is termed the com- 

 plemental air. These phenomena have been accurately observed 

 only in man, but the essential conditions are the same in the cat. 



6. It has just been said that all vocal manifestations are 

 modified expiratory actions ; but these actions, in order that they 

 should produce sounds, need the aid of a peculiar mechanism. This 

 mechanism is furnished by that expanded, anterior end of the 

 windpipe which has been already referred to as the LARYNX. 



To the upper, anterior margin of the larynx, the hyoid bone is 

 attached, and therefore also the tongue, behind the root of which, is 

 the laryngeal opening into the pharynx the " glottis " already- 

 spoken of as situated in front of the cesophageal opening, and as being 

 protected by that cartilaginous process the epiglottis, which stands 

 up in front of it. The larynx is formed of three large and two small 

 cartilages, united together by fibrous tissue moved by muscles, and 

 supplied with blood- vessels, lymphatics, and nerves, the whole structure 



