RESPIRATION. 



581 



air in respiration in the smaller mammals may be recorded by connecting 

 the trachea by means of a tube with Marey's tambour: 



This instrument consists of a cylindrical box, the upper surface of which is 

 formed by a sheet of rubber membrane, the interior being connected by tubing 

 with the trachea, and a lever so adjusted to the rubber membrane as to record. 

 its movements (Fig. 252). If air is forced into this box it will, of course, produce 

 bulging of the rubber membrane, and thus cause the ascent of the lever, while, 

 on the other hand, if the air be rarefied in the interior of the box the membrane 

 will be depressed and the lever descend. By allowing such a lever to record its 

 movements on some revolving surface, as, for example, on the smoked paper 

 of a kyinographion, if the interior of the box be connected with the trachea of an 

 animal, namely, in the dog or rabbit, a curve somewhat similar to the following 

 will be produced (Fig. 253). 



The descents in this curve represent the inspirations, the ascents the 

 expirations. It is seen that the curve of inspiration begins suddenly 

 and advances rapidly, and is then succeeded by an expiratory movement, 

 which at first is more rapid than inspiration, but gradually becomes 

 slower. No pause is present between the end of inspiration and the 

 beginning of expiration, but a short pause is noted at the end of expi- 



FIG. 253. TRACING OF THORACIC RESPIRATORY MOVEMENTS. (Foster.) 



(To be read from left to right.) 



A whole respiratory phase is comprised between a and a, inspiration extending from a to b, and ex- 

 piration from b to a. The undulations at c are caused by the heart's beats. 



ration, the undulations in the curve at this point being caused by the 

 heart's beats. 



The duration of inspiration is, as a rule, shorter than expiration, the 

 proportion being from ten to fourteen, although this is not invariable. 

 The pause which is noted at the end of expiration will average in dura- 

 tion about one-fifth to one-third the time occupied by the entire respira- 

 tory movement. 



The number of respirations in most animals may be placed, as a 

 rule, as one respiratory movement to four pulsations of the heart. Ex- 

 ercise and a large number of other conditions will greatly increase the 

 number of respiratory movements by increasing the amount of tissue 

 change, and, therefore, increasing the amount of carbon dioxide in the 

 blood which has to be eliminated in respiration. 



In cattle the rate of respiration is higher in cows than in bulls or 

 steers. During sleep the average rate in the cow is twenty-two in the 



