THE BLOODVESSELS. 



691 



Fisr. 287 



trace out definite courses for the capillaries, so that they sometimes 

 present elongated meshes, sometimes rounded, narrower or wider reticu- 

 lations. The physiological energy is still more influential, and it is a 

 general rule, that the greater the activity of an organ, whether as 

 regards contractions or sensations, excretion or absorption, so much the 

 closer is the capillary network, and 

 so much the more abundant the 

 supply of blood. The capillary 

 plexuses are closest in the secernent 

 and absorbent organs, as in the 

 glands, above all in the lungs, liver, 

 and kidneys ; next in the integu- 

 ments and mucous membranes ; much 

 wider in the organs which receive 

 blood only for the purpose of their 

 own nutrition, as the muscles, nerves, 

 organs of sense, serous membranes, 

 tendons, and bones ; although among 

 these organs differences exist, as, 

 for instance, the muscles, and the 

 gray nerve-substance, are more abun- </ 

 dantly supplied than the other parts 

 above enumerated. The diameter 

 of the capillaries themselves is al- 

 most directly in an inverse rela- 

 tion, and they have the thinnest 

 walls and are smallest (0-002-0-003 

 of a line) in the nerves, muscles, retina, and the Peyerian patches ; in 

 the external integument and mucous membranes they attain the size of 

 0-003-0-005, in the glands and bones, lastly, one of 0-004-0-006 of 

 a line, and in the compact substance of the latter, although no 

 longer having, in all respects, the structure of capillaries, even the 

 diameter of 0-0080*01 of a line. Physiology is not as yet in a con- 

 dition to explain these differences in all particulars, inasmuch as infor- 

 mation is wanting with respect to the laws of diffusion in the various 

 capillary membranes ; and also because the more minute conditions of 

 the sanguineous circulation, in the separate organs, are wholly un- 

 known. 



The mode in which the capillaries pass into the larger vessels is diffi- 

 cult of investigation. On the arterial side it is found that the capil- 



FIG. 287. Finest vessels on the arterial side of the capillaries. 1, an artery of the 

 smallest size ; 2, transitional vessel; 3, coarser capillary; 4, finer capillary : a, structureless 

 coat, with a few nuclei, representing the t. adventitia ; 6, nuclei of the muscular fibre-cells : 

 c, nuclei within the minute artery, probably still belonging to an epithelium; d, nuclei of the 

 capillaries and transitional vessels. From the human brain ; magnified 300 diameters. 



