PER1PHERIC ORGANS OF THE CIRCULATION. 159 



they excite the contraction of the small arteries, and thus 

 diminish their size : cold, especially, serves to produce con- 

 traction, as may be verified in the mesentery of a frog 

 (experiment by Schwann) ; in this case the small arteries 

 diminish to one-seventh of their calibre (Fig. 54). In the 

 normal state the arterial muscle is sometimes contracted, and 

 sometimes relaxed ; but, taking into account the variations in 

 size, and the changes in the circulation resulting from them, 

 we can in this only see rhythmical contractions calculated to 

 assist those of the heart. The arterial muscle takes no active 

 part in the pulsation : in this phenomenon it is simply pas- 

 sive, as we have already shown. 



Capillaries. The diameter of these vessels is extremely 

 small : in the smallest it is hardly sufficient to allow of the 

 passage of a blood globule; the size, however, differs in 

 different parts. The capillaries of the skin are large in com- 

 parison with those of the lungs or of the brain, and, on 

 account of the size of the capillaries of the fingers, it is easy 

 to inject through the arteries the commencement of the veins 

 of the foot or of the hand. 



The capillaries are generally formed of coats of very simple 

 structure : their tissue is apparently amorphous, but traces of 

 cellular structure are found in them, in the shape of laminated 

 flattened plates, the remains of ancient cells, which have lost 

 the principal physiological properties of the globular element 

 when losing its form. The capillaries haVe not, however, 

 perhaps, always distinct walls : this is probably the case with 

 the capillaries of the liver, which are, apparently, only lacunae 

 hollowed out in the substance of this organ (interstices be- 

 tween groups of hepatic cells). 



What we have already seen shows that, in general, the 

 circulation is continued in the capillaries, and that the car- 

 diac wave is felt in them only under exceptional circum- 

 stances. We have also studied and explained the presence 

 of what is called the inert layer (see Fig. 43, above). 



The capillaries are not contractile : their structure forbids 

 our attributing this property to them, and all the phenom- 

 ena of dilatation or contraction which we observe in them 

 are entirely passive, owing to similar phenomena taking 

 place in the small arteries and veins. The ancient physiolo- 

 gists believed with Bichat that the function of the capillaries 

 was active, and that they are very contractile ; they considered 

 them as forming a peripheral heart. Glisson's capsule, a 

 fibrous tissue surrounding the vascular network of the liver, 



