CAPILLARIES. 161 



hoeck, Prochaska, and a crowd of others; yet there are anato- 

 mists who hold a contrary doctrine, and admit the parenchyma 

 of the ancients (an indefinable something, conceived, however, to 

 be spongy) as a point of termination for the arteries, ancKof 

 commencement for the veins. 



Though the capillaries are all too fine to be seen distinctly 

 without the microscope, yet they are found to have several gra- 

 dations of size. The largest of them are those which only 

 escape the naked eye, experience successive divisions, whereby 

 their diameters are reduced from admitting a file of several 

 globules of blood to the caliber of one globule only.* The 

 capillaries have also frequent anastomoses with one another. 

 Sometimes the artery is simply doubled on itself, and immedi- 

 ately becomes a vein: on other occasions, several capillary ar- 

 teries run into the same vein. When these communications 

 are unduly enlarged, they constitute what has been called by 

 Mr. John Bell the aneurism from anastomosis, a frequent mark 

 in young children, and which, when it has developed itself 

 fully, has a spongy structure resembling the erectile tissues, as 

 the corpus cavernosum penis, &c. As there is a double circu- 

 lation, so there is a double capillary system, one for the lungs 

 and the .other for the body generally: to these may be added a 

 third, which exists in the liver, between the hepatic extremi- 

 ties of the vena portarum and the hepatic veins. 



The texture of the capillary vessels is too fine to admit of 

 much scrutiny, but they appear as simple cylindrical excava- 

 tions in the substance of the part to which they belong. It is 

 not improbable, that they may be Uninterrupted continuations 

 of the internal coat of the arteries into that of the veins. They 

 have striking powers of extension and of contraction, and are 

 easily irritated. An emotion of the mind, as a sentiment of 

 shame or a feeling of resentment, quickly causes those of the 

 face to become turgid with blood. Local stimuli cause con- 

 gestions in them. Cold, the application of a weak acid, or 

 fear, causes them to contract; though, under the influence of 

 the heart, they are less so than larger vessels. Their innume- 

 rable channels cause a comparatively languid circulation of 

 the blood in thorn, for reasons mentioned; and by furnishing \( 



* Beclard, loc. cit. 



