142 



ANIMAL MECHANICS. 



resistances offered in man and in the horse, to the capillary 

 circulation, we could estimate the relative forces with which 

 the blood is propelled in the circulation of these animals. 



The experiments of Poiseuille on the discharge of liquids 

 through capillary tubes, prove that the resistance offered by 

 such tubes is directly proportional to the lengths of the tubes, 

 and inversely proportional to the squares of their cross sec- 

 tions. 



The quantity of liquid discharged by a capillary tube in a 

 given time is inversely proportional to the resistance, and may 

 be expressed in general by the following formula — 



Q = Ax -j-. (18) 



[n this expression, Q denotes the quantity of liquid discharged 

 in a given time ; A is a constant ; h denotes the charge, or 

 hydrostatical pressure of the fluid ; and d and I are the diame- 

 ter and length of the capillary tube. 



There is reason to believe that in animals similar in bulk, 

 the arrangement and structure of the capillaries are such, 

 that the ratio of the squares of their cross sections to the total 

 lengths of the capillaries, is practically constant ; as may be 

 shown by a comparison of the sheep and the dog ; — the data 

 concerning which animals I take from Dr. Hales' observations 

 and experiments. The left ventricle of the heart of the sheep 

 contains 1.85 cubic inches, and its pulse beats 65 times in a 

 minute ; the quantity of blood passing through the capillaries 

 in a given time being proportional to the product of these two 

 quantities. The haemastatical pressure of the blood in the 

 large arteries was found by Dr. Hales to be 6.46 ft. of blood. 



Equation (18) gives us, for the coefficient depending on 

 the capillary resistance, 



I h 6.40 



