138 ANIMAL PHYSIOLOGY. 



be expected to exercise a certain influence on the arteriefc 

 and veins, drawing in and pressing out the blood in them, in 

 the same way as the air is drawn in and pressed out through 

 the windpipe; and this is actually the case, so far as the 

 veins are concerned, the flow of blood into the chest being 

 greater in inspiration; but the blood in the arteries is pro- 

 tected from the effects of inspiration by the strength of the 

 arterial coats, and thus inspiration assists the circulation by 

 its effect on the veins, without retarding the blood in the 

 arteries. The respiratory rhythm consists of three parts 

 the inspiration, the expiration (which has only half the 

 duration of the inspiration), and a period of rest after the 

 expiration; and it has been found that the arterial pressure 

 begins to rise when inspiration is begun, and increases during 

 expiration, then falls in the period of rest (Sanderson). 



101. The enlargement of the thoracic cavity in inspiration 

 is accomplished partly by the diaphragm, partly by movement 

 of the ribs. The diaphragm is the floor of the thoracic 

 cavity, separating it from the abdomen. It is a muscle, 

 tendinous in the centre, and with muscular fibres radiating 

 all round, and inserted into lumbar vertebrae, into the car- 

 tilages of the lower six ribs, and into the lower end of the 

 breast bone. When, at rest it is arched upwards, in a dome 

 over the liver and stomach, and lies against the costal walls 

 of the thorax for some distance behind and at the sides ; but 

 when it is contracted, its arched fibres are straightened 

 and drawn asunder from the thoracic walls, pressing down- 

 wards the viscera beneath it, and producing a compensatory 

 heaving of the abdomen. Diaphragmatic or abdominal breath- 

 ing occurs in a very marked degree in children, the floor of 

 the thorax bearing a much larger proportion to its height in 

 children than in the adult. 



The breathing of women is sometimes called superior costal, 

 and that of men inferior costal, because the most obvious move- 

 ments of the ribs are above the breasts in women, and in men 

 on the sides of the chest. The elevation of the ribs will be best 

 observed lay placing the hands on the sides and front of the 

 chest of another person while he inhales deeply. The hands 

 will then be felt to be raised and separated one from the 

 other. If the breast bone be watched, it will be seen that its 



