310 SPECIAL HISTOLOGY. 



they sometimes present elongated meshes, sometimes rounded, 

 narrower or wider reticulations. 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 integuments 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 grey nerve-substance, are 

 more abundantly supplied than the other parts above enume- 

 rated. The diameter of the capillaries themselves is almost 

 directly in an inverse relation, and they have the thinnest 

 walls and are smallest (0*002 — 0003"') in the nerves, muscles, 

 retina, and Peyerian patches ; in the external integument and 

 mucous membranes they attain the size of 0003 — 0*005'", in 

 the glands and bones, lastly, one of 0004 — 0006'", and in the 

 compact substance of the latter, although no longer having, in 

 all respects, the structure of capillaries, even the diameter of 

 0008 — 0-01"'. Physiology is not as yet in a condition 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 sepa- 

 rate organs, are wholly unknown. 



The mode in which the capillaries pass into the larger 

 vessels is difficult of investigation. On the arterial side it is 

 found, that the capillaries, as they become wider, present more 

 closely placed nuclei, and are then invested externally with a 

 structureless /. adventitia, and solitary muscle-cells, whence, 

 when they have reached the diameter of 0007'", they already 

 exhibit the aspect of the finest arteries (fig. 287, 1 ). After- 

 wards, the nuclei seem to be replaced by epithelium cells, whilst 

 the capillary membrane either ceases or is continuous with the 

 elastic inner membrane. The venous transitionary vessels 

 are less characteristic for a greater length. The first thing 



