660 RESPIRATION AND MUSCULAR WORK. [BOOK n. 



more directly conscious of changes in the former than of changes 

 in the latter. Hence when the organism ceases to be able to 

 meet the demands which the labour is making upon it, the 

 subject is said to be " out of breath," though in a large, number of 

 cases the failure lies much more at the door of the vascular than 

 of the respiratory system. And, as a rule, it may perhaps be said 

 that when two men differ in their capacity for strenuous work, 

 such as running a race, the difference, though it is often familiarly 

 spoken of as one of " wind " or power of breathing, is in reality not 

 a difference in ventilating capacity but a difference in the power 

 of the heart to keep up to and work in harmony with the increased 

 respiratory movements. 



Thus there are two main factors in respiration, the respiratory 

 mechanism proper, and the circulation, the one bringing the air to 

 the blood, and the other the blood to the air. We may remind 

 the reader that there is also a third factor, and that one of great 

 moment, the amount of haemoglobin in the blood. The amount 

 of oxygen taken up from the lungs depends not only on the strokes 

 of the respiratory and the vascular pumps but also on the richness 

 of the blood in haemoglobin; and this is determined by the number 

 of red corpuscles and by the quantity of hemoglobin in each. 

 A body which from loss of blood or from disease is anaemic 

 is thrown out of breath by very slight exertion, not so much 

 because the respiratory or the vascular pump is weak, but because, 

 through lack of oxygen carriers, with their best efforts the com- 

 bined pumps can only deliver to the tissues, including the spinal 

 bulb, an inadequate supply of oxygen. And fat persons, whose 

 store of haemoglobin in proportion to their body weight is always 

 below par, are proverbially " scant of breath." 



