. Animal physiology. Physiology, Comparative. 220 CAPILLARY nuous with the commencing twigs of the Venous system. The communication is established by means of a set of extremely minute vessels, which are termed Capillaries.'^ These capil- laries form a network, Avhich is to be found in almost every part of the body (fig. 121). It is in them alone that the blood ministers to the opera- tions of nutrition and secre- tion. Even the walls of the larger blood-vessels are inca- pable of directly imbibing nourishment from the blood which passes through them; but are supplied with minu
. Animal physiology. Physiology, Comparative. 220 CAPILLARY nuous with the commencing twigs of the Venous system. The communication is established by means of a set of extremely minute vessels, which are termed Capillaries.'^ These capil- laries form a network, Avhich is to be found in almost every part of the body (fig. 121). It is in them alone that the blood ministers to the opera- tions of nutrition and secre- tion. Even the walls of the larger blood-vessels are inca- pable 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 capillaries 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 1 - 2500th to 1-1600th of an inch in dia- meter. In the true capilla- ries, 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 considerable difference in the diameter of the same capillary at different ; a change sometimes taking place from causes which are not yet The rate at which the blood moves * From the Latin capilla, hair ; so named on account of their being, like hairs, of very minute size. Their diameter is really, however, far less than that of ordinary hairs. t The circulation of the blood in the Frog's foot, the tail of the Tadpole, the gilis of the larva of the Water-Newt, the yolk-bag of embryo Fish, and other appropriate subjects for the observation, is one of the most beautiful and interesting spectacles that the Microscope can open to us. Details of the various modes of exhibiting it will be found in the Author's treatise on " The Microscope and its Eevelations," Chap, xviii. Fig. 121.—Portion of thi
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