VESSELS. ARTERIES. VEINS. CAPILLARIES, ETC. 81 



their prolongations, with the walls of capillaries 

 already in existence, and thus contribute to the for- 

 mation of the capillary plexus, or network. Their 

 prolongations, increasing in diameter, ultimately 

 become true capillary vessels, and the body of the 

 cell itself corresponds to the point of confluence of 

 several vascular canals. The same author asserts that 

 many large vessels are formed out of capillaries, by 

 the transformation of the cells which surround them 

 into their several tunics. 



BLOOD AISTD LYMPH. In a histological point of view j^ p d h and 

 the composition of blood and lymph is exceedingly 

 simple. The organized elements of the blood are of 

 two species, red and white globules. The red glo- 

 bules are bi-concave discs measuring, on an average, 

 aajth to 3i<rth of aline in breadth, and Worth to TTO o-th 

 of a line in thickness. Their external envelope is so 

 exceedingly delicate that it seems to be continuous 

 with their contents. They consist internally of an 

 amorphous substance of some density, very elastic, 

 and colored yellowish red by hsematine. When 

 exposed to the air these globules become rapidly 

 altered in form, and as a common rule, show wrinkles, 

 or indentations, upon their surfaces (PL I. fig. I. 3). 

 The red globules, according to Schmidt, constitute 

 'about one half of the whole mass of the blood. 



The white globules of the blood differ from the white giobuiea. 

 preceding, both in size and shape. They are sphe- 

 rical corpuscles, with rough or tuberculated surfaces, 

 and average in diameter from 2<rth to rioth of a line 

 (PI. I. fig. I. 4). Their contents, granular and trans- 

 parent, include, sometimes, a nucleus so large as to 



