PECULIARITIES OF THE CIRCULATION. 191 



although the quantity of blood contained, at any instant, 

 in the greater circulation of the body, is far greater than 

 the quantity within the lesser circulation ; yet, in any given 

 space of time, as much blood must pass through the lungs 

 as passes in the same time through the systemic circulation. 

 If the systemic vessels contain five times as much bloo d as 

 the pulmonary, the blood in them must move five times as 

 slow as in these ; else, the right side of the heart would be 

 either overfilled or not filled enough. 



Peculiarities of the Circulation, in different Parts. 



The most remarkable peculiarities attending the circula- 

 tion of blood through different organs are observed in the 

 cases of the lungs, the liver, the Irain, and the erectile organs. 

 The pulmonary and portal circulations have been already 

 alluded to (pp. in, IT 2), and will be again noticed when 

 considering the functions of the lungs and liver. 



The chief circumstances requiring notice, in relation to 

 the cerebral circulation, are observed in the arrangement and 

 distribution of the vessels of the brain, and in the con- 

 ditions attending the amount of blood usually contained 

 within the cranium. 



The functions of the brain seem to require that it should 

 receive a large supply of blood. This is accomplished 

 through the number and size of its arteries, the two internal 

 carotids, and the two vertebrals. But it appears to be 

 further necessary that the force with which this blood is 

 sent to the brain should be less, or at least, subject to less 

 variation from external circumstances than it is in other 

 parts. This object is effected by several provisions ; such 

 as the tortuosity of the large arteries, and their wide anas- 

 tomoses in the formation of the circle of Willis, which will 

 insure that the supply of blood to the brain may be uni- 

 form, though it may by an accident be diminished, or 

 in some way changed, through one or more of the principal 

 arteries. The transit of the large arteries through bone, 



