300 



FUNCTIONS OF NUTRITION. 



The capillary bleod-vessels are most abundant, and connected by the 

 closest inosculations in organs where the blood serves for other pur- 

 poses than local nutrition ; such as aeration, secretion, or absorption. 

 One of the finest capillary networks is that of the lungs, in which the 

 diameter of the intervascular spaces is sometimes a little greater and 

 sometimes a little less than that of the capillaries themselves. In the 

 intra-lobular plexus of the liver, they are only a little wider than the 

 vessels forming the network. In the nerves, the serous membranes 

 and the tendons, on the other hand, the capillaries are less closely 

 interwoven ; and in the adipose tissue they form wide meshes, em- 

 bracing the fat vesicles. 



Movement of the Blood in the Capillary Vessels. The motion of 

 the blood through the capillaries may be studied, under the microscope, 

 in any transparent tissue of sufficient vascularity. The frog is the 

 most convenient animal for this purpose, owing to the readiness with 

 which the circulation will go on in the exposed organs at ordinary tem- 

 peratures. To secure immobility, the brain and medulla oblongata 

 should be broken up by a needle introduced through the cranium, or 

 the voluntary muscles paralyzed by the subcutaneous injection of six 

 drops of a filtered watery solution of woorafa, made in the proportion 

 of one part to five hundred. The body should be enveloped in a loose, 

 moistened bandage, to prevent desiccation. The tongue, the web of 

 the foot, the pulmonary membrane, the mesentery or the bladder may 

 be used to exhibit the capillary circulation, which, with the aid of 

 appropriate mechanical appliances, may be maintained in either of these 

 regions for several hours. 



When examined in this man- 

 ner, the smaller arterial ramifi- 

 cations, the capillary vessels, 

 and the minute veins are often 

 visible in the same tissue. The 

 blood can be seen entering the 

 field by the arteries, shooting 

 through them with great rapid- 

 ity in successive impulses, and 

 flowing off more slowly by the 

 veins. In the capillary plexus 

 it moves with a uniform cur- 

 rent, considerably less rapid 

 than in either the arteries or 

 the veins. A further peculi- 

 arity of the capillary circula- 

 tion is that it has no definite 

 origin or termination, but rep- 

 resents a movement of the blood through all parts of the tissue. Its 

 streams pass indifferently above and below, at right angles to each 



FIG. 78. 



CAPILLARY CIRCULATION in web of frog's foot. 



