220 
CAPILLARY BLOOD-VESSELS. 
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, which 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 
times ; a change sometimes taking place from causes which 
are not yet understood, f 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. 
f The circulation of the blood in the Frog’s foot, the tail of the 
Tadpole, the gills 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 Revelations,” 
Chap, xviii. 
Fig. 121.— Portion of the Membrane 
BETWEEN THE TOES OF THE HIND 
foot of the Frog, more highly 
magnified than in fig. 120, showing the 
network of Capillaries that traverses it; 
a, small venous trunk ; b b, branches 
communicating with the capillaries ; 
c, intervening tissue covered with epi¬ 
thelium cells. 
