THE CAPILLARY TISSUE. 85 



The precise point of separation is not determined; hence 

 this tissue is viewed as a system of vessels belonging neither 

 to the arteries nor the veins, "but one sui-generis, and, ac- 

 cording to Bichat, independent in its action. 



The microscope reveals the capillary tissue to consist 

 of a multitude of very minute, hair-like vessels, having 

 distinct parieties, and assuming the form of a net-work. 

 Though this is the general form, there is found to be some 

 variation from the different sizes of the meshes, and from 

 their being elongated or not. The capillary arrangement 

 in the small intestines, according to Soemmering, is arbor- 

 escent; in the muscles a bunch of twigs; in the tongue a 

 hair pencil; in the liver a star; in the schneiderian mem- 

 brane a trellis-work; in the choroid plexus of the brain, 

 and testicle, a lock of hair; in the kidneys glomerulate. 

 The capillaries are the minutest tubes in the body, con- 

 sisting, says Beclard, of thin, soft, transparent walls; and, 

 according to Muller, having a mean diameter of 1.3700 to 

 1.1850 of an inch, allowing sufficient space for the free 

 passage of the blood globules, which are only from the 

 1.5000 to the 1.3000 of an inch. 



The structure of the capillaries is regarded by Schwann 

 as fibrous, the same as the larger vessels, and their con- 

 tractile power experiments demonstrate to be far greater. 



The capillaries are not equally abundant in every por- 

 tion of the body. The quantity is estimated by injections, 

 congestions and inflammations. An opinion prevailed that 

 the whole body was made up of blood-vessels, from the very 

 minute injections of the celebrated Ruysch. The micro- 

 scope, however, shows that various parts of the body are 

 more vascular than others, and that there are some en- 

 tirely destitute of any vessels whatever. The mesentery, 

 or the web of the frog's foot under the microscope, pre- 

 sents its most minute capillaries, those admitting but one 

 globule, as separated by a considerable space, while in the 

 mucous membrane, belonging to the organs of respiration, 

 in the same animal, it has been observed to be impossible 

 to stick a very fine needle without opening several of these 



