370 



MANUAL OF HISTOLOGY. 



with arteries for the nutrition of their walls. These are known as the 

 vasa vasorum, and are distributed, for the most part, to the middle and 

 external coats, and especially to the latter, in which they are tolerably 

 numerous. They are here arranged like those of formless connective- 

 tissue, except that they form narrower meshes. Later on they appear in 

 the middle layer, where they have been seen in 

 arteries to form vascular networks of fine tubes, 

 with elongated oblique meshes (Gerlach). 



The nerves of arteries derived from the sympa- 

 thetic and cerebro-spinal system are distributed in 

 the larger trunks to the middle and external tunics. 

 As a rule, the arteries seem to be richer in nerves, 

 on account of their thicker middle layer, than the 

 veins, but the greatest variety exists in this respect. 

 All that is necessary has already been remarked in 

 respect to the termination of the nerves of vessels 

 in 183. 



Fig. 363. Vessels of striped 

 muscle, a, artery; 6, vein; 

 c and d, extended capil- 

 lary network. 



205. 



"We mus now turn to the more careful considera- 

 tion of the capillaries as to the most important 

 subdivision of the vascular system. 



We have already seen that no sharp boundary 

 can be drawn between these vessels and the arteries 

 and veins, in that the most imperceptible transition 

 exists from one to the other. But one thing is char- 

 acteristic in the capillaries, namely, that their* tubes 

 no longer decrease in calibre from the giving off of 

 branches, and that they form among themselves, in 

 the various organs supplied by them, networks of 

 tolerably regular size and shape (fig. 363, c, d). 

 The diameter of the vessels so connected is, how- 

 ever, by no means the same in the various organs, 

 and the finest are not presented to us in every locality. The brain and 

 retina possess the most delicate. Their transverse diameter in these organs 



may be stated at 0'0068-0'0065 mm., 

 or, in some cases, even so low as 

 0*0056 mm. In muscle they appear 

 to be somewhat larger, measuring 

 0'0074 mm. Those of the connec- 

 tive-tissue of the skin and mucous 

 membranes, again, are still larger. In 

 most glands, as, for instance, the liver, 

 kidneys, and lungs, the diameter of 

 the capillaries lies between 0'0091 

 and 0-0135 mm. The largest of all 

 vessels of this kind are to be found 

 in bony tissue, where they measure as 

 high as 0-0226 mm. It must be 

 borne in mind, however, that, owing 

 to the elasticity of the capillary tube, 

 and its variation in diameter according to the amount of blood contained 

 in it, these measurements can only be regarded as approximate. 



Fig. 364. A pulmonary^alveolus from the calf. 

 a, large blood-vessel " 6, capillary network; 

 c, epithelium cells. 



