176 



BLOOD-VESSELS. 



larger ones like the veins ; but the diameter of the tubes, as well as 

 the shape and size of the reticular meshes which they form, differs 

 in diflFerent textures. Their prevalent size in the human body may, 

 speaking generally, be stated at from 3-^0 to -joVo of ^^ i^ich, as 

 measured when naturally filled with blood. But they are said to be 

 in some parts considerably smaller, and in others larger than this 

 standard : thus, Weber has measured injected capillaries in the 

 brain, which he found to be not wider than tjj^-^ of an inch, and 

 Henle has observed some still smaller, — in both cases apparently 

 smaller than the natural diameter of the blood-corpuscles. The 

 capillaries, however, when deprived of blood, probably shrink in 

 calibre immediately after death ; and this consideration, together 

 with the fact that their distension by artificial injection may exceed 

 or fall short of what is natural, should make us hesitate on such 

 evidence to admit the existence of vessels incapable of receiving the red 

 particles of the blood. Tlie diameter of the capillaries of the marrow, 

 or of the medullary membrane, is stated as high as -j-aW of an inch. 

 In other parts, their size varies between these extremes : it is small in 

 the lungs, small also in muscle ; larger in the skin and mucous mem- 

 branes. The extreme branches of the arteries and the commencing veins 

 in certain parts of the synovial membranes are connected by capillary 

 loops, which are considerably dilated at their point of flexure. 



There are differences also in the size or width of the meshes of the 

 capillary network in different parts, and consequently in the number of 

 vessels distributed in a given space, and the amount of blood supplied 

 to the tissue. The network is very close in the lungs and in the choroid 

 coat of the eye, close also in muscle, in the skin, and in most mucous 



Fig. 116. 



Fig. 117. 



Fig. 116. — Injected Capillary Vessels of Muscle, seen -mTH a low magnifying 



POWER. 



Fig. 117. — Network of Capillary Vessels op the Air-Cells op the Horse's Lung, 



MAGNIFIED. 



a, a, capillaries proceeding from 6, h, terminal brandies of the pulmonary artery- 

 after Frey). 



membranes, in glands and secreting structures, and in the grey part 



