Chap. 23.] Structure of tie Blood Vejjels. '241 



veins, however, have mufcular coats in all animals, is 

 inferred from their contractile power. 



The venous fyftem is far more capacious than the 

 arterious. 



Arteries are commonly faid to diminifh in iize, as 

 they recede from the heart j but this is not the real ftate 

 of the cafe. As long as an artery continues undivided, 

 its diameter remains the fame ; and when it does di- 

 vide, the area of the veflels formed by this divifion is 

 always greater than the area of the artery from which 

 they are produced j fo that the artery may in truth be 

 faid to be increafed. This rule holds equally with 

 refpect to the divifion of the great trunks of arteries, 

 and the fub-divifions of their branches. The trunks 

 alfo of veins are always fmaller than the fum of the 

 fmaller veins from which they are formed. 



The larger trunks of blood- veflels are feparate tubes, 

 but their branches form various communications with 

 each other, and thefe communications increafe as the 

 veflels become more minute, fo as at length to form a 

 web of veflels in the parts on which they are diftribute.d. 

 The advantages of this ftrudure are very obvious, as 

 by a communication of veflels each part may receive 

 blood from many fources, and no part therefore fuffers 

 by the divifion of the blood-veflel which more parti- 

 cularly belongs to itj its advantages are like thofe of 

 commerce among mankind, by which the effects of 

 partial lofles are guarded againft by a mutual exchange 

 of conveniencies. 



The branches of arteries are in general fent off at 

 much more acute angles than thofe of the veins, by 

 which the paflage of the blood through the arteries is 

 the lefs impeded. 



The arteries have in general a correfponding vein 

 placed near themj but to this rule 'there are ieveral 



VOL, IK; R excep- 



