352 Dr. A. du Pre Denning and Mr. J. H. Watson. [May 7, 



number of corpuscles than for blood containing few (cf. curves in 

 Curve-Table 9). 



8. The addition of certain chemical reagents decreases the viscosity, whilst 

 other substances may increase it. 



An Indication of the Bearing of Viscosity upon the Circulation of the Blood. 



In the next few pages we wish very briefly to indicate the import of 

 such results as the foregoing in any consideration of the mechanism of the 

 circulation of the blood. For this purpose we may with advantage regard 

 the human circulatory system from a simple mechanical point of view, 

 as a well-arranged though very complex net-like labyrinth of elastic pipes, 

 namely, the arteries and veins, .connected with a central pumping-station, 

 the heart, which maintains a constant circulation of a viscous fluid, having 

 a peculiar and variable consistency — the blood ; and to this simplified 

 circulation endeavour to apply the above conclusions, when alterations occur 

 affecting the whole or any one part of this system ; for it is evident at once 

 that any disturbance in the mechanism may arise from — 



1. Conditions that interfere with the normal activity of the heart ; 



2. From alterations in the calibre of the blood vessels : 



3. From changes in the amount or the composition of the blood with the 



consequent variations in viscosity. 



Effect of Temperature. — For instance, it is well known that in ordinary 

 healthy persons, as a result of vigorous exercise, the temperature may rise 

 to 101° F., or even higher, and there is associated with this condition of 

 hyperthermia an increased frequency of the heart beat, and at the same time 

 dilatation of the peripheral blood vessels. The result of these changes is to 

 increase the velocity of the blood flow : firstly, because the increased 

 temperature will give rise in itself to a diminution in viscosity ; and secondly, 

 the increased calibre of the vessels reduces the peripheral resistance. As u 

 nett result, there is a more rapid filling of the heart, but for each individual 

 beat there is less resistance to be overcome, and consequently the heart is 

 saved an appreciable strain ; that is to say, we have a simple illustration of 

 nature's way of mechanically adjusting the balance of the circulatory system 

 so as to compensate for the extra work thrown upon the heart. 



Passing from this normal physiological reaction we are led to consider the 

 conditions met with in fever. In the early stages, with a raised temperature 

 and a rapidly beating heart, there is an undoubted increase in the peripheral 

 resistance (Maragliano)* and, moreover, the composition of the blood is 

 altered — there being invariably a slight leucocytosis and an addition to the 

 * Maragliano, ' Zeit. f. Kl. Med.,' vol. 14, p. 309, 1888. 



