162 NUTRITIONAL PHYSIOLOGY 



wall between the lymph and the passing blood offers prac- 

 tically no impediment to the movement. It will be re- 

 membered that the blood which enters upon the very short 

 journey through the capillaries is arterial; that which 

 enters the minute veins only a fraction of an inch away is 

 reckoned venous. It has parted with a large share of its 

 oxygen and has received carbon dioxid. It is swept on 

 without significant change to the right side of the heart 

 and thence to the lungs. The development of these organs 

 is such as to multiply the surface of contact between the 

 blood and the air within them. The capillaries of the 

 pulmonary circulation are wrapped about innumerable 

 elastic sacs, the walls of which are as thin as those of the 

 capillaries themselves. There is, accordingly, a double 

 partition between the blood and the air, but it is of a nature 

 to permit free gaseous exchange. 



If the air in the sacs were not renewed it would accumu- 

 late carbon dioxid in increasing amount, while its oxygen 

 would progressively diminish. This tendency is normally 

 counteracted through the effects of breathing. The lungs 

 have no power to move of themselves; the changes which 

 they undergo are due to the widening and the return of the 

 thoracic walls. This is not the place to analyze the breath- 

 ing movements. The muscles employed are of the skeletal 

 order. Being so, they are not automatic, and it follows 

 that every breath taken is the expression of a separate and 

 distinct act on the part of the central nervous system. 

 Each time the chest is made larger the air presses in along 

 the breathing passages to fill the space created for it in 

 the host of widened sacs. The return of the chest walls 

 to their first position reduces the capacity of the sacs, and 

 air is pressed out along the same channels by which it 

 entered. The action is that of a pair of bellows not pro- 

 vided with the usual inlet valve. 



It is not to be conceived that we empty and refill the 

 air spaces of the lungs with each breath. We usually expel 

 something like one-fifth or one-sixth of the air contained 

 and replace that fraction with fresh air. When allowance 



