THE BLOODVESSELS 83 



while if the same vessel be divided the loss of blood would be 

 nearly 1 pint per second. 



The Duration of the Circulation depends upon the length of 

 time it takes a red corpuscle to travel from a given point and 

 back to it again.* But there are many different paths it can 

 take. For instance, from left heart through coronary vessels to 

 right heart and again back to left heart would occupy a shorter 

 time than a course through the liver, or through the feet or tail, 

 so that a circulation time may mean nothing more than that a 

 certain number of corpuscles have found the shortest cut through 

 the circulation, or, on the other hand, have taken the longest. 

 In a horse with a pulse frequency of 42, the average complete 

 circuit is performed in 313 seconds (Hering), and is equivalent, 

 according to the latter observer, to about 28 beats of the heart. 

 In the rabbit with a pulse frequency of 168 per minute, the 

 time occupied in completing the round of the circulation was 

 779 seconds, or, again, in 28 heart-beats ; with the dog 167 

 seconds or in 267 heart-beats. 



Stewart, who introduced the method of electrical conductivity 

 for ascertaining the duration of the circulation, states that the 

 time occupied by the blood in passing through the kidney, spleen, 

 and liver is relatively long and much more variable than that 

 in the lungs. In a dog weighing 13 3 kilogrammes the average 

 circulation time in the spleen was 1095 seconds, kidney 13-3 

 seconds, lungs 8*4 seconds. The same observer found the circu- 

 lation time of the stomach and intestines of the rabbit to be 

 comparatively short, not exceeding that of the lungs, while the 

 retina and coronary vessels of the heart had the shortest time 

 circulation. 



Influence of the Nervous System. — It is only during the last 

 sixty years that the existence of a set of nerve fibres governing 

 the calibre of the bloodvessels has been known. C. Bernard 

 observed that in the cervical sympathetic of rabbits there were 

 fibres which on stimulation produced constriction of the blood- 

 vessels of the ear ; later he found a set of fibres which on stimula- 

 tion produced a dilatation of the bloodvessels. 



The control of the nervous system over the bloodvessels lies 

 in these two directions — viz., dilatation and constriction ; the 

 nerves inducing the former are called vaso- dilators, and those 

 bringing about the latter vaso-constrictors. Collectively both 

 nerves are known as vasomotor. The vaso-constrictor fibres 

 act by causing contraction of the muscular coat of the arterioles. 



* The circulation time is determined either by injecting an easily- 

 distinguishable salt into the blood, or more precisely by increasing the 

 electrical conductivity of the blood by injecting into it a neutral salt 

 solution. 



