SMALLER ARTERIES, VEINS AND CAPILLARIES. 369 



the mode in which the passage takes place was not ascertained until some time after 

 the date of his great discovery. The finding of the capillary vessels, and of the 

 course of the blood through them, was in fact one of the first fruits of the use of 

 the microscope in anatomy and physiology, and was reserved for Malpighi (in 1661). 

 When the web of a frog's foot is viewed through a microscope of moderate 

 power (as in fig. 427), the blood is seen passing rapidly along the small arteries, and 

 thence more slowly through a network of finer channels, by which it is conducted 

 into the veins. The small vessels interposed between the finest branches of the 

 arteries and the commencing veins, are the capillary vessels. The course of the 



Fig. 427. CAPILLARY BLOOD-VESSELS IN THE WEB OF A FROG'S 

 FOOT, AS SEEN WITH THE MICROSCOPE (after Allen Thomson). 



The arrows indicate the course of the blood. 



blood in them may be conveniently seen also in the 

 lungs or mesentery of the frog, in the external gills 

 and tail of tadpoles ; in the tail of small fishes ; in 

 the mesentery of small quadrupeds ; and generally, 

 in short, in the transparent vascular parts of animals 

 which can be brought under the microscope. These 

 vessels can also be demonstrated by means of fine 

 injections of coloured material, not only in mem- 

 branous parts, such as those above mentioned, but also in more thick and opaque 

 tissues, which can be subsequently rendered transparent. 



The capillary vessels of a part are most commonly arranged in a network, the 

 branches of which are of nearly uniform size, though not all strictly equal ; and 

 thus they do not divide into smaller branches like the arteries, or unite into 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 different textures. Their prevalent 

 size in the human body may, speaking generally, be stated at from -g^o to -^Vo- f 

 an inch, 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 measured injected capillaries in the brain, which he found to be not wider 

 than iTbtr of an inch, and Henle has observed some still smaller, in both cases 

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

 laries, 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 snch evidence to admit the existence of vessels incapable of permitting 

 the red corpuscles of the blood easily to pass through them. The diameter of the 

 capillaries of the marrow of the bones is stated to be a ^Vo f an i ncn - ^ n other 

 parts, their size varies between the extremes mentioned : it is small in the lungs, 

 and in muscle ; larger in the skin and mucous membranes. The extreme branches 

 of the arteries and veins in certain parts of the synovial membranes are connected 

 by capillary loops, which are considerably dilated at their point of flexure, and 

 dilatations are also found upon the transverse capillaries of the red muscles of the 

 rabbit. 



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, and comparatively close in 

 muscle, in fat, in the skin, and in most mucous membranes, also 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, 



