GENERAL CONSIDERATIONS. 155 



the lesser or the pulmonary circulation; and that which begins 

 at the left ventricle, goes through the whole body, and ends in 

 the right auricle, the greater circulation. 



The blood contained in the veins of the greater circulation,-in 

 the right auricle and ventricle, and in the pulmonary artery, is 

 of a dark brown or reddish colour; while that contained in the 

 pulmonary veins, in the left auricle and ventricle, arid in the 

 aorta and its ramifications, is, from being vivified by respira- 

 tion, of a carmine or vermilion complexion. The celebrated 

 Bichat has, upon this difference of colour, founded his division 

 of the whole circulating system into two parts; one containing 

 black blood, "Systeme vasculaire a sang noire;" the other red 

 blood, " Systeme vasculaire a sang rouge." This division 

 having general physiology for its object, affords a well marked 

 distinction, suited to such discussions. 



The lymphatics also are a part of the circulatory system, but 

 as they do not commonly convey red blood, the consideration 

 of them will be introduced subsequently. " They take a very 

 active part in the animal economy, whether natural or diseased, 

 and seem, in many actions, to be the antagonists of the arteries, 

 while the veins are much more passive, being principally em- 

 ployed in returning the blood to the heart."* 



The largest vascular trunks are situated near the centre of the 

 body and limbs, on the side upon which flexion is accomplished, 

 while those near the surface are generally small. Most com- 

 monly there are one artery, one or two veins, and several lym- 

 phatics, all together. 



The arterial system in its external configuration may be com- 

 pared to a tree, the trunk of which is attached to the heart, and 

 which by a continued succession of divisions and subdivisions 

 reaches to every part of the body. There are no means of esti- 

 mating rigidly the collective area of the branches in proportion 

 to that of the trunk, but a little observation on the size of the 

 primitive branches will satisfy one of a great excess on the part 

 of the latter; and as the rule is maintained throughout, there 

 must finally be an immense disproportion. We have then rea- 

 son to believe, that if all the branches were assembled into a 

 single cavity, this cavity would be somewhat like a cone, the 



* Hunter, loc. cit. 



