240 Vafcular Syftem. [Book IX. 



The velTels of the human body are either blood- 

 veflels or lymphatics. 



The blood- veflels are membranous tubes, which 

 convey the blood to and from the various parts of the 

 body. They are divided into arteries and veins. The 

 arteries pulfate, and convey the blood from the heart j 

 the veins return it towards the heart, and do not 

 pulfate *. The large trunks, both of the arteries and 

 veins, are near the heart ; at a diftance frojn it they 

 are divided into numerous fmall branches in a manner 

 Very fimilar to that in which the trunk of a tree is 

 loft in its branches and twigs. 



The arteries are formed by the following tunics. 

 The firft is derived from the cavity, through which 

 the artery pafles ; in the thorax, from the pleura - t 

 in the abdomen, from the peritoneum, &c. The 

 fecond is a loofe covering of cellular fubftancc, which 

 contains fmaller veffels, for the nourifhment of that 

 on which they run, and which in the large arteries 

 often contains a confiderable quantity of fat. The 

 third is mufcular, and is compofed of feveral fmall 

 arches of maifcular fibres, many of which go to the 

 formation of a circle. Within -this is a thin cellular 

 coat, 'which adheres clofely to the former; and laftly, 

 there is a firm, fmooth, and whitifli coat, with which 

 the circulating mafs of fluids is in contact. 



The ftrudhire of the veins is the fame as that of 

 the arteries, but more delicate. The mufcular coat 

 is in them fo thin, or of fo pale a colour, as not to 

 .admit of demonflration in man, but is plainly feen in 

 a veffel called the vena portarum of the ox. That 



* As a pulfe is only to be perceived in the arteries, this circum- 

 ftance will enable the moft unfkilful to diftinguiih the nature of 

 any blood -veffel. 



veins, 



