328 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



the ! ^ O th of an inch, to -j-J^th, according to the measurements 



of Weber. 



The finest capillaries are found in the brain (fig. 195 A) and in 



the retina ; those of muscles, especially the cross branches which 



intersect the fibres, are likewise very fine. Among the largest are 



those of the lung and liver. 



The capillaries form a net- work in each tissue or organ, 



which derives its nourishment directly from them, and they 



exhibit an arrangement adapted to the 

 disposition of its proximate elements. 

 In the tissues which assume a fibrous 

 form, as muscle, nerve, fibrous tissue, 

 the capillaries are disposed in lines 

 parallel to the fibres, and these parallel 

 vessels are united at variable intervals 

 by cross branches, which pass at right 

 angles to the fibres. (See figs. 47, 95, 

 161, 162, and 194). In compound or 

 involuted mucous membranes, the capil- 



Arrangement of the capillaries on the laries form a pleXUS with more Or less 

 mucous membrane of the large intestine , , 1-1 i 



m the human subject. Magnified 50 circular meshes, which correspond in 



form arid size to the' arrangement of the 



membrane good examples of this are found in the mucous mem- 

 brane of the stomach and of the intestines (fig. 194). When the 

 elements of the mucous membrane are prolonged into processes, 

 forming villi or papillae, each villus or papilla is found to possess its 

 plexus or system of minute capillary vessels (figs. 161 and 162). 

 In the simple mucous membranes, and in serous membranes, the 

 plexus of capillaries placed in the submucous or subserous areolar 

 tissue exhibits large and irregular meshes. 



In the compound tissues, the capillaries have no direct relation 

 to the ultimate anatomical elements that is to say, these mi- 

 nute vessels do not ramify among the ultimate particles of the 

 tissues. It is with their proximate elements they connect them- 

 selves. Thus in muscle the vessels lie between the fibres, and are 

 separated from the sarcous particles by the sarcolemma : in nerve, 

 in the same way, they are separated from the nervous matter 

 by the tubular membrane ; and in the vesicular matter they play 

 around the vesicles and do not penetrate them. In most of the 

 mucous membranes, the basement membrane is placed between 

 them and the epithelium, their nidus being the sub-basement tissue. 

 So, albo, with the serous membranes. In bone the finest vessels 



