134 



1LAKD-BOOK OF PHYSIOLOGY. 



Capillaries are surrounded by a delicate nerve-plexus resembling, in 

 miniature, that of the larger blood-vessels. 



The diameter of the capillary vessels varies somewhat in the different 

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

 inch. 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 relation 

 to the size of their blood corpuscles: thus, in the Proteus, the capillary 

 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 modi- 

 fications of two chief kinds of mesh, the rounded and the elongated. That 



FIG. 116. 



FIG. 117. 



FIG. 116. Network of capillary vessels of the air-cells of the horse's lung magnified, a, a, cap- 

 illaries proceeding from &, ft, terminal branches of the pulmonary artery. (Frey.) 



FIG. 117. Injected capillary vessels of muscle seen with a low magnifying power. (Sharpey.) 



kind of 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. 116), most glands, and mucous mem- 

 branes, and the cutis. The meshes of this kind of network are not quite 

 circular but more or less angular, sometimes presenting a nearly regular 

 quadrangular or polygonal form, but being more frequently irregular. 

 The capillary network with elongated meshes (Fig. 117) 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 usually have 

 the form of a parallelogram, the short sides of which may be from three 

 to eight or ten times less than the long ones; the long sides always corre- 

 sponding to the axis of the fibre or tube, by which it is placed. The ap- 

 pearance of both the rounded and elongated meshes is much varied 



