300 RESPIRATION. 



into the lungs coincides with the closure of the nares, and is 

 determined by the approximation of the body of the hyoid 

 bone to the roof of the phaiynx, and that the expulsion of air 

 from the lungs by the contraction of the flanks occurs while 

 the hyoid is still drawn upwards, so that the two muscular 

 movements form part of the same act. 



87. External Respiratory Movements of Man and 

 Mammalia. The alternate emptying and filling of the air 

 cells of the lungs, which is the final cause of respiration, is 

 effected by the alternate enlargement and contraction of the 

 chest. If the whole of the thorax were occupied by the air 

 cells, these changes of capacity could be measured by the quan- 

 tity of air entering and leaving the respiratory cavity in each 

 act of breathing. As, however, in addition to the lungs, the 

 chest contains various other organs, some of which alter their 

 volume very considerably, according to the degree of expansion 

 of the cavity in which they are contained, there is no constant 

 relation between the enlargement or diminution of the available 

 intra-thoracic air space and the external enlargement or dimi- 

 nution of the thorax. 



There is no practicable method of determining the changes 

 of volume which the chest undergoes in respiration with exacti- 

 tude. As, however, the imperfect methods we possess differ 

 from most of those employed in physiology, in being quite as 

 applicable to man as to the lower animals, and are sufficiently 

 accurate to yield valuable results in the study of disease, they 

 are well worthy of the attention of the pli3 r sician, though of 

 comparatively little interest to the physiologist. 



88. The external movements of the human chest maybe in- 

 vestigated by recording the variations either of its diameters 

 or of its circumference, at different parts, or of both simulta- 

 neously. For the graphic measurement of the circumference, 

 an instrument contrived by Marey, and much improved by 

 Bert, is used. It consists of an air-tight cylinder of brass and 

 India-rubber, of the shape and construction of a common drum, 

 the cylinder being of brass and the membranous ends of India- 

 rubber. The cylinder communicates by a flexible tube with a 

 tympanum, the lever of which records its variations of capacity. 

 To the centre of each of the two terminal membranes a metal 

 disk is attached, which is furnished with a hook, and is thus 

 connected with one of the ends of an inelastic cincture, which 

 encircles the circumference to be measured. As the circum- 

 ference augments, the membranes are extended, and the ca 7 

 pacity of the drum increased, and vice versa. It is obvious 

 that before the instrument is used it must be graduated. The 

 mode of accomplishing this will be given further on. 



89. The graphic measurement of the diameters of the chest 

 is much more simple, inasmuch as it merely involves the trans- 



