CIRCULATORY SYSTEM. 165 



all the blood out of the body, the animal sinks and dies long- 

 before its vessels are evacuated ; and as we possess no means of 

 measuring what remains behind, any calculation we may make 

 from the quantity that has flowed must necessarily turn out vague, 

 if not allogether incorrect. The following experiment will serve 

 to illustrate this; at the same time, that its result may serve as a 

 datum to guide us in any future computation. 



The weight being ascertained to be 791b, a puncture was 

 made with a lancet into the jugular vein, from which the blood, 

 which flowed in a very free stream, was collected. The vein 

 having ceased to bleed, the carotid artery of the same side 

 was divided, but no blood came from it : in a few seconds after- 

 wards the animal was dead. The weight of the carcass was 

 now found to be 73^1fe; consequently, it had sustained a 

 loss of 5ilb, precisely the measure of the blood drawn. It 

 appears from this experiment that an animal will lose about 

 l-15th part of its weight of blood before it dies ; though a less 

 quantity may so far debilitate the vital powers as to be, though 

 less suddenly, equally fatal. In the human subject the quantity of 

 blood has been computed at about 1-Sth part of the weiglit of tiie 

 body; and as such an opinion has been broached from the results 

 of experiments on quadrupeds, we may fairly take that to be 

 about the proportion of it in the horse : so that, if we estimate 

 the weight of a common sized horse at about 12 cwt., the whole 

 quantity of blood will amount to 84qrts., or 168 Ife, of which about 

 45 qrts., or 90 tfe, will commonly flow from thejugular vein prior to 

 death ; though the loss of a much less quantity will sometimes de- 

 prive the animal of life*. It is well known that young animals 

 possess more blood than old, and that they will, perhaps, on 

 this account sustain greater bodily injuries, and bear larger 

 haemorrhages ; indeed, they are wisely provided with such an ex- 

 cess, if we may so term it, in order that their growth may be 

 promoted, and their several organs maintained in a state of vi- 

 gour : but in old, in which the body is gradually decaying, and 

 the powers of life declining, the quantity of this fluid becomes re- 

 duced. Mr. Wilson, in his Lectures on the Blood, See, says, 



* Supposing a man to weigh 12 st., or 168 lb, the quantity of blood con- 

 tained in his body may be rated at 21 lb, or 2 gall. 2 qrts. and 1 pint. Again, 

 granting that a dog weighs 40 Iti, the amount of his blood will be 5 pints. 

 These calculations are useful and worth our attention, inasmuch as they 

 serve to guide us in practice, as to the probable extent to which we may, 

 with safety, carry venesection in ditFerent animals. For instance, we may 

 reckon the loss of a pint from a man to be equivalent to that of a gallon 

 from a horse, or to four ounces from a dog, -AnA vice versa ; selecting in- 

 <lividuals from each class about the respective wciglits we have here set 

 down. 



