PHYSIOLOGY. 165 



(CLXX.) take place more or less in the part. 3. As the 

 gravity of the blood concurs with, or opposes its motion in the 

 part. 4. As causes increasing or diminishing the action of the 

 arteries of the part are applied or removed, 



CLXXVIII. The quantity of blood distributed to any par- 

 ticular part of the sanguiferous system, will be greater or less 

 according to the velocity and impetus of the blood in the part, 

 by CLXX VI I. and according to the resistances in other 

 parts being increased or diminished by constriction, compres- 

 sion, ligature, position, relaxation, or aperture. 



CLXXIX. The flexibility and contractility of the blood- 

 vessels render the effects of all increase or diminution of resis- 

 tance in any particular part most considerable in the nearest, 

 and very little so in the more remote vessels of the system. By 

 this we are to judge of the celebrated doctrines of derivation and 

 revulsion. 



CLXXX. The quantity of blood distributed to the different 

 parts of the system, is in different proportion at different periods 

 of life. 



1. The capacity and force of the heart, in proportion to the 

 system of vessels, is greater at the beginning of life than at any 

 after period. Till the body arrives at its full growth, the ca- 

 pacity of the vessels increases in greater proportion than that 

 of the heart ; but, from that period, the capacity of the vessels 

 is constantly diminishing, while that of the heart suffers little 

 change. 



2. A greater quantity of blood is contained in the arteries, in 

 proportion to that which is contained in the veins, at the begin- 

 ning of life than at any after period. From the time that the body 

 has arrived at its full growth, the quantity of blood contained in 

 the veins, in proportion to that which is contained in the arte- 

 ries, is constantly increasing. " This difference of the quanti- 

 ties of blood in the arteries and veins manifestly occurs in the 

 ordinary progress of life ; but it is also probable, that in some 

 persons the same difference, to a certain degree, takes place 

 through the whole course of life, and gives a constant and con- 

 siderable difference in the temperaments of men, as I shall men- 

 tion more fully hereafter." M. M. 



3*. The vessels of the head receive a greater quantity of blood 



