THE CAPILLARIES. 223 



belong to the second class in about one-third of the cases ex- 

 amined. In the other arteries that form is much more rare. The 

 vessels which present the cylindrical form most frequently, are 

 the radial and crural arteries. The first form, that of a truncated 

 cone, with the base to the heart, is by far most frequent in the 

 arteries of the limbs. The popliteal, however, always presents 

 the fifth form, that of two truncated cones opposed by their 

 basis, or in other words is expanded in its middle at the place 

 where popliteal aneurism usually occurs. 



Of the Capillaries. 



The extreme ramifications of the arteries, constitute the 

 capillary blood-vessels, which are intermediate to, and form 

 the connexion between the arteries and veins, and may be 

 considered as the radicles of the latter vessels. The extreme 

 minuteness and delicacy of structure of the capillary vessels, 

 are too great to admit of any very rigid scrutiny into the struc- 

 ture of their walls, but it is considered probable that the internal 

 coat of the arteries, is extended through them, so as to be con- 

 tinuous with that of the veins. The capillary vessels, were at 

 one time considered as having other terminations besides those 

 which connected them with the arteries, on one hand, and the 

 veins on the other : some were considered, as opening directly 

 into, and thus forming the secretory ducts of the different glands ; 

 others as ending in a manner nearly similar, in order to form an 

 imaginary system of vessels, called the exhalents, in the skin 

 and the different serous and mucous membranes ; and others, 

 according to Lond and Lepelletier de la Sarthe, as terminating 

 in the midst of the different tissues, so as to allow the mole- 

 cules of blood to escape naked among the organized parts, for 

 the purposes of nutrition. It is now, however, generally admit- 

 ted, that the capillaries have no termination but by their anas- 

 tomoses with each other, and their communication with the arte- 

 ries and veins ; that all the fluids which pass from them, for 

 the purposes of nutrition, secretion, or exhalation, permeate in 

 some manner their delicate coats, by an action, somewhat like 

 the exosmosis of Dutrochet. 



