174 A MANUAL OF PHYSIOLOGY . 



pended vertically with the feet down, the blood drains into the 

 abdominal vessels, syncope speedily ensues, and in a period that 

 ranges from less than a quarter to three-quarters of an hour the 

 animal dies in the convulsions of acute cerebral anaemia (Salathe, 

 Hill). The head-down position has no ill effects. In wild rabbits, 

 whose abdominal wall is more tense and elastic, these fatal symp- 

 toms are not easily produced, and the same is true of cats and 

 dogs. But in all animals, when the compensation is destroyed, 

 as in paralysis of the vaso-motor centre by chloroform, the cir- 

 culation may be profoundly influenced by the position of the 

 body : elevation of the head may lead to cerebral anaemia, 

 syncope, and even death ; elevation of the legs, and particularly 

 the abdomen, may restore the sinking pulse by filling the heart 

 and the vessels of the brain. If a chloralized dog be fastened 

 on a board which can be rotated about a horizontal axis passing 

 under the neck, the blood-pressure in the carotid artery falls 

 greatly when the animal is made to assume the vertical position 

 with the head up, and either rises a little or remains practically 

 unchanged when the head is made to hang down. So great may 

 the fall of pressure be in the former position that death may 

 occur if it be long maintained (Practical Exercises, p. 199). 



Finally, it is in virtue of the amazing power of accommoda- 

 tion possessed by the vascular system, as controlled by the 

 vaso-motor and cardiac nerves, that so long as these are not 

 disabled the total quantity of blood may be greatly diminished 

 or greatly increased, without endangering life, or even causing 

 more than a transient alteration in the arterial pressure. It is 

 not until at least a quarter of the blood has been withdrawn 

 that there is any notable effect on the pressure, for the loss is 

 quickly compensated by an increase in the activity of the heart 

 and a constriction of the small arteries. An animal may recover 

 after losing considerably more than half its blood.* Conversely, 

 the volume of the circulating liquid may be doubled by the 

 injection of blood or physiological salt solution without causing 

 death, and increased by 50 per cent, without any marked in- 

 crease in the pressure. The excess is promptly stowed away 

 in the dilated vessels, especially those of the splanchnic area ; 

 the water passes rapidly into the lymph, and is then more 

 gradually eliminated by the kidneys. 



From these facts we can deduce the practical lesson, that blood- 

 letting, unless fairly copious, is useless as a means of lowering 



* It is not usually possible to obtain quite two-thirds of the total blood 

 by bleeding a dog from a large artery. In seven dogs bled from the carotid 

 in the laboratory of the writer, the ratio of the weight of the blood obtained 

 to the body-weight was i : 24*7, i : 21-7, i : 20*7, i : 2O'6, i : i8'6, i : 16, 

 1:13-5. In the last case, the blood clotted with abnormal slowness, and 

 the animal died in a few minutes. 



