THE CIRCULATION OF THE BLOOD. 



Ill 



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 rela- 

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

 capillary circulation, can just be discerned with the naked eye. 



The form 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 elongated. 

 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 net- 

 work is most dense, such as the lungs (Fig, 106), most glands, and 

 mucous membranes, and the cutis. The meshes of this kind of network 



FIG. 106. Fio. 107. 



FIG. 106. Network of capillary vessels of the air-cells of the horse's lung magnified. 



capillaries proceeding from 6, 6, terminal branches of the pulmonary artery. < Frey .) 

 FIG. 107. Injected capillary vessels of muscle seen with a low magnifying 



a, a, 



(Sharpey.) 



are not quite circular, but more or less angular, sometimes presenting a 

 nearly regular quadrangular or polygonal form, but being more fre- 

 quently irregular. The capillary network with elongated meshes (Fig. 

 107) 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 corresponding to the axis of the fibre or tube, by 

 which it is placed. The appearance of both the rounded and elongated 

 meshes is much varied according as the vessels composing them have a 

 straight or tortuous form. Sometimes the capillaries have a looped 

 arrangement, a single capillary projecting from the common network 



