500 CAPILLARY VESSELS. [SECT. 2l6. 



cording as they lie among one or other of these elementary tissues ; 

 thus they present sometimes more elongated meshes, sometimes 

 roundish, narrower, or wider networks. The physiological energy 

 of the parts is still more important, as determining the arrange- 

 ment of the capillary plexuses ; and it is a general law, that the 

 greater the activity of an organ, whether expressed in contractions 

 or sensations, secretion or absorption, the denser are the capillary 

 networks, and the more abundant is the quantity of blood supplied 

 to it. The capillary networks are closest in the organs of secretion 

 and absorption, as in the glands, above all in the lungs, the liver, 

 and the kidneys ; next in the skin and mucous membranes ; they 

 are much wider in the organs which receive blood only for the 

 sake of their own nourishment and for no other object, as in the 

 muscles, nerves, organs of the senses, serous membranes, tendons, 

 and bones ; yet even here differences occur ; the muscles and the 

 grey nervous substance, for example, being much more abundantly 

 supplied than the other parts mentioned. The diameter of the 

 capillaries themselves present almost exactly a converse condition ; 

 for they are finest (0-002'" to o - oo3'"), and their walls are thinnest, 

 in the nerves, muscles, in the retina, and in the Peyerian follicles; 

 next, in the mucous layer of the skin, and in the mucous mem- 

 branes, they amount to 0-003'" to , 005" / in diameter; in the 

 glands and bones, lastly, they reach a diameter of 0-004'" to 

 0'Oo6'"; and in the compact substance of the latter, although no 

 longer possessing quite the structure of capillaries, they measure 

 even 0-008'" and o-oi'". Physiology is" not in a position to explain 

 all these differences in detail, through the deficiency of our know- 

 ledge of the laws of diffusion through the different capillary mem- 

 branes, and also because the finer modifications of the circulation 

 in the several organs are completely unknown to us. 



The manner in which the capillaries pass into the larger vessels 

 is difficult to follow. Towards the arteries, the capillaries, as they 

 become broader, receive more closely disposed nuclei, and then 

 become covered externally with a structureless tunica adventitia 

 and with separate muscular cells; and in this manner they already 

 have the appearance of the narrowest arteries when they have 

 reached a diameter of 0-007'" (fig. 204, 1). Epithelial cells then 

 appear to take the place of the nuclei, while the capillary mem- 

 brane either disappears, or becomes continuous with the elastic inner 

 coat. The venous vessels of transition do not become characteristic 

 for a longer distance. The first structure that is here superadded 

 to the capillary membrane is an outer, homogeneous, nucleated 



