336 RESPIRATION. 



varies largely, the average may be put down at 3000-4000 c.c. (200 to 

 250 cubic inches). 



Of the whole measure of vital capacity, about 500 c.c. (30 cubic inches) 

 may be put down as the average amount of tidal air, the remainder being 

 nearly equally divided between the complemental and reserve airs. The 

 quantity left in the lungs after the deepest expiration amounts to about 

 1400 to 2000 c.c. 



Since the respiratory movements are so easily affected by various circumstances, 

 the simple fact of attention being directed to the breathing being sufficient to cause 

 modifications both of the rate and depth of the respiration, it becomes very diffi- 

 cult to fix the volume of an average breath. Thus various authors have given 

 figures varying from 53 c.c. to 792 c.c. The statement made above is the mean 

 of observations varying from 177 to 699 c.c. 



271. Graphic records of respiratory movements. These may be ob- 

 tained in various ways. 



The simplest, readiest, and perhaps the most generally useful method is that of 

 recording the movements of the column of air. This may be effected by intro- 

 ducing a T-piece into the trachea, one cross-piece being left open and the other 

 connected with a Marey's tambour or with a receiver, which in turn is connected 

 with a tambour (see Fig. 54 and Fig. 90). The movements of the column of air 

 in the trachea are transmitted to the tambour, the consequent expansions and 

 contractions of which are transmitted to the recording drum by means of a lever 

 resting on it. 



If, a receiver being used, the open end of the I be closed, the animal breathes 

 into and out of the receiver, and the movements of the tambour are greatly in- 

 creased. This has the disadvantage that the air in the receiver soon becomes 

 unfit for further respiration. A similar increase of the movements of the lever 

 of the tambour may be obtained by connecting a piece of India-rubber tubing to 

 the open end of the I. By increasing the length of this tube, or slightly con- 

 tracting it, the movements of the lever may be increased without very seriously 

 interfering with the breathing of the animal. 



In another method the movements of the chest are recorded. When a small 

 animal, such as a rabbit, is used, the whole animal may be placed in an air-tight 

 box, breathing being carried on by means of a tube inserted into the trachea and 

 carried through an air-tight orifice in the wall of the box. By another orifice 

 and tube the air in the box is brought into connection with a tambour, which 

 accordingly registers the changes of pressure in the air of the box produced by 

 the movements of the chest (and body), and thus indirectly the movements of 

 the chest. In man and larger animals the changes in the girth of the chest may 

 be conveniently recorded by means of Marey's pneumograph. This consists of a 

 hollow elastic cylinder, or a cylinder with elastic ends, the interior of which is 

 connected with a tambour. By means of a strap attached to each end of the 

 cylinder the instrument can be buckled round the chest like a girdle. When the 

 chest expands, the ends of the cylinder are pulled out, and the air within the 

 chamber rarefied; in consequence the lever of the tambour connected with its 

 interior is depressed ; conversely, when the chest contracts, the lever is elevated. 

 The pneumatograph of Fick is somewhat similar. Or changes in one or other 

 diameter of the chest may be recorded by what maybe called the "calipers" 

 method, as in the recording stethometer of Burdon-Sanderson. This consists of 

 a rectangular framework constructed of two rigid parallel bars joined at right 

 angles to a cross-piece. The free ends of the bars, the distance between which 

 can be regulated at pleasure, are armed, the one with a tambour, the other simply 

 with an ivory button. The tambour bears on the metal plate of its membrane 

 (m', Fig. 54) a small ivory button in place of the lever. When it is desired to 

 record the changes occurring in any diameter of the chest, e. c/., an antero-pos- 

 terior diameter from a point in the sternum to a point in the back, the instrument 

 is made to encircle the chest somewhat after the fashion of a pair of calipers, the 

 ivory button at one free end being placed on the spine of a vertebra behind and 

 the tambour at the other on the sternum in front in the line of the diameter which 



