clxxvi 



BLOOD-VESSELS. 



Fig. XCIV. 



Fig. XCIV. CAPILLARY BLOOD- 

 VESSELS IN THE WEB OP A FROG'S 

 FOOT, AS SEEN WITH THE MICRO- 

 SCOPE (after Dr. Allen Thom- 

 son). 



The arrows indicate the course of 

 the blood. 



different textures. Their prevalent size in the human body may, speaking 

 generally, be stated at froin^ 1 ^ to ^^ of an inch, as measured when 

 naturally filled with blood. But they are said to be in some parts consider- 

 ably 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 ^-yo-o" f 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 in- 

 jection 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 par- 

 ticles of the blood. The diameter of the 

 capillaries of the marrow, or of the medul- 

 lary membrane, is stated as high as -J-^QQ 

 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 membranes. 

 According to Mr. Toynbee, the extreme 

 branches of the arteries and the com- 

 mencing veins in certain parts of the synovia! membranes are connected by 

 loops of vessels, which are dilated at their point of flexure to a greater 

 size even than the vessels which they immediately connect. 



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 parts of the 

 mucous membrane, in glands and secreting structures, and in the grey part 

 of the brain and spinal cord. On the other hand, it has wide meshes and 

 comparatively few vessels in the ligaments, tendons, and other allied tex- 

 tures. In infants and young persons, the tissues are more vascular than in 

 after-life ; growing parts, too, are more abundantly supplied with vessels 

 than those which are stationary. 



The figure of the capillary network is not the same in all textures. In 

 many cases the shape of the meshes seems accommodated to the arrange- 

 ment of the elements of the tissue in which they lie. Thus in muscle, nerve, 

 and tendon, the meshes are long and comparatively narrow, and run con- 

 formably with the fibres and fasciculi of these textuies (fig. xcv.). In 

 other parts the meshes are rounded or polygonal, with no one dimension 

 greatly predominating (fig. xcvi.). In the smaller-sized papillae of the skin 

 and mucous membranes, the vessels of the network are often drawn out 

 into prominent loops. 



Structure of the small- sized vessels and capillaries. The capillary vessels 

 have real coats, and are not mere channels drilled in the tissue which they 

 pervade, as has sometimes been maintained. In various parts they are 

 readily separable from the surrounding substance, as in the brain and 



