. Animal physiology. Physiology, Comparative; Physiology, Comparative. 196 CAPILLARY Fig. 114.—Capillaries in the web of the Frog's foot ; a, arteries, v, veins. a network, which is to be found in almost every part of the „ a body. It is in them alone that the blood ministers to the operations of nutrition and secretion. Even the walls of the blood-vessels are incapable of directly imbibing nourishment from the blood which passes through them ; but are supplied with minute branches, which proceed from neighbouring trunks, and form a capillary network in their substance. The diameter
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 196 CAPILLARY Fig. 114.—Capillaries in the web of the Frog's foot ; a, arteries, v, veins. a network, which is to be found in almost every part of the „ a body. It is in them alone that the blood ministers to the operations of nutrition and secretion. Even the walls of the blood-vessels are incapable of directly imbibing nourishment from the blood which passes through them ; but are supplied with minute branches, which proceed from neighbouring trunks, and form a capillary network in their substance. The diameter of the capil- laries must of course bear a certain proportion to that of the blood-discs which have to pass through them. In Man they are commonly from about l-2500th to 1-1600th of an inch in dia- meter. In the true capillaries, it would seem that only one row or file of these particles can pass at a time; but we frequently see vessels passing across from the arteries to the veins, which will admit several rows. There seems, however, to be a con- siderable difference in the diameter of the same capillary at dif- ferent times ; a change sometimes taking place from causes which are not yet understood. This system of capillary vessels evi- dently bears a strong resemblance to that of the laticiferous ves- sels of Plants (See Veget. Phys. §. 87) ; but in the latter, there are no large trunks, because there is no central organ of impulsion corresponding to the heart of Animals, and the circu- lation is entirely capillary. 252. The Arterial and Venous systems thus communicate with each other at their opposite extremities ;—their large trunks through the medium of the heart;—and their ultimate subdi- visions through the capillaries. Hence we may consider this double apparatus of vessels as forming a complete circle, through which the blood flows in an uninterrupted stream, returning continually to its point of departure ; and the term circulation is therefore strictly applic
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