134 



THE CIRCULATION. 



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 



FIG. 50. 



FIG. 51. 



FIG. 50. Network of capillary vessels of the air-cells of the horse's lung, magnified 

 a, a, capillaries proceeding from 6, 6, terminal branches of the pulmonary artery 

 (after Frey). 



FIG. 51. Injected capillary vessels of muscle, seen with a low magnifying power 

 (Sharpey). 



the fibre or tube, by which it is placed. The appearance of 

 both the rounded and elongated meshes is much varied accord- 

 ing 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 into 

 some prominent organ, and returning after forming one or 

 more loops, as in the papillae of the tongue and skin. What- 

 ever be the form of the capillary network in any tissue or 

 organ, it is, as a rule, found to prevail in the corresponding 

 parts of all animals. 



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

 different parts determine in general the degree of vascularity 

 of those parts. The parts in which the network of capillaries 

 is closest, that is, in which the meshes or interspaces are the 

 smallest, are the lungs and the choroid membrane of the eye. 

 In the iris and ciliary body the interspaces are somewhat 

 wider, yet very small. In the human liver, the interspaces 

 are of the same size, or even smaller than the capillary vessels 



