184 HANDBOOK OF PHYSIOLOGY. 



The diameter of thp capillary vessels varies somewhat in the different 

 textures of the body, the most common size being about -oVoth of an 

 inch, IS;*. Among the smallest may be mentioned those of the brain, 

 and of the follicles of the mucous membrane of the intestines; among 

 the largest, those of the skin, and especially those of the medulla of 

 bones. 



The size of capillaries varies necessarily in different animals in rela- 



Fig. 157. Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothelial 

 membrane of which they are composed. (Klein and Noble Smith.) 



tion to the size of their blood corpuscles : thus, in the Proteus, the capil- 

 lary circulation can just be discerned with the naked eye. 



The/orm of the capillary network presents considerable variety in 

 the different textures of the body: the varieties consisting principally 

 of modifications of two chief kinds of mesh, the rounded and the elon- 

 gated. That kind in which the meshes or interspaces have a roundish 

 form is the most common, and prevails in those parts in which the 

 capillary network is most dense, such as the lungs (fig. 158), most 

 glands, and mucous membranes, and the cutis. The meshes of this 

 kind of network are not quite circular but more or less angular, some- 

 times presenting a nearly regular quadrangular or polygonal form, but 

 being more frequently irregular. The capillary network with elongated 

 meshes is observed in parts in which the vessels are arranged among 

 bundles of fine tubes or fibres, as in muscles and nerves. In such parts, 

 the meshes form parallelograms, the short sides of which may be from 

 three to eight or ten times less than the long ones; the long sides being 

 more or less parallel to the long axis of the fibre. The rounded and 

 elongated meshes vary according as the vessels composing them are 

 straight or tortuous. 



The number of the capillaries and the size of the meshes in different 

 parts determine in general the degree of vascularity of those parts. 



