688 />K/\C//*LES OF GENERAL PHYSIOLOGY 



in grams, n approximately , and k a constant varying with the particular species. 

 In other words, the blood volume is a function of the surface and would be 



2 



exactly proportional to W* if the animal were spherical. The formula also 

 implies that smaller individuals of the same species have a relatively greater 

 volume of blood than the larger ones. 



This relation is obviously an important fact when doses of toxin or drugs are 



to be given and it is required to know their concentration in the blood. It may 



therefore be useful to give the values of the blood constant, k, as far as determined: 



Tame rabbit . 1-58 



Guinea-pig . 3-30 



Mouse - - 6-70 



Hare - - 094 



Wild rabbit . 9-Q4 



Wild rat - . 3-Q5 



(Dreyer and Ray, 1911, p. 198.) 



The relative dosesr of drugs for adults and children can also be calculated on 

 a more accurate basis than heretofore. 



THE REGULATION OP BLOOD SUPPLY 



If the arterioles of an organ are caused to dilate, the volume of the blood 

 flowing through the organ is increased, and the pressure in the capillaries raised. 

 Coincidently, the peripheral resistance is decreased, so that, if the region in which 

 the dilatation occurs is a considerable fraction of the whole circulation, the aortic 

 pressure falls, unless the heart beat is increased to compensate for it. 



Methods of Investigation. In order to measure the state of the circulation in 

 an organ, we may take tracings of the changes in its volume, due to greater or 

 less distension of its blood vessels, by some plethysmographic method. 



In this method the organ is enclosed in an air-tight box, provision being made that 

 the nerves and blood vessels are not compressed, and the interior of the box is connected 

 to some instrument which records by its movement the amount of air sent into or removed 

 from the recorder as the organ alters in volume. It has been suggested that changes in 

 general venous pressure would interfere with the correct interpretation of the results. In 

 actual fact, it has been found that experiments by the plethysmographic method and by 

 determination of the actual rate of flow of blood give the same results. Of course, due account 

 must be taken of changes in the general arterial pressure, which alters the rate of flow apart 

 from local changes. If, for example, along with fall in arterial pressure, the organ decreases 

 in volume, no information is obtained as to any active changes in the blood vessels of the 

 organ itself. But, if the organ expands with a fall of arterial pressure, no other interpretation 

 is possible than that its blood vessels have actively dilated. If the organ contracts, with 

 a fall of arterial pressure, we cannot draw the conclusion that local vaso-dilatation is absent, 

 because it may be too small to counteract the effect of the general fall of pressure in draining 

 away blood. 



The second method is one in which the blood flow from the vein of an organ 

 is estimated. If it comes in drops, these may be intercepted by falling on to 

 a lever which actuates a signal, electric or pneumatic. If more copious, it may 

 be measured in a "tipper," such as that described by Condon (1913), or iii 

 a siphon outflow recorder, such as that of Ishikawa and Starling (1912) or of 

 Gunn (1913). 



Details of various methods will be found in Frank's article (1913). 



Vasomotor Nerves. The supply of the smooth muscle of the arterioles by 

 both excitatory and inhibitory nerves has been referred to above (page 403) in 

 the general discussion of excitation and inhibition. 



The name "vasomotor" should be applied to both classes, since both bring 

 about movement of the vessel wall. Confusion is sometimes caused by using 

 " vasomotor " as equivalent to " vaso-constrictor." 



The first clear proof of the existence of vaso-constrictor nerves was afforded, 

 independently, by Brown-Sequard (1852) and by Claude Bernard (1852). They 

 found that the blood vessels in the region supplied by the cervical sympathetic 

 nerve were constricted when the peripheral end of the nerve was excited. 



