THE CAPILLARY VESSELS. 
5 
comparatively slight in structure, but the ventricles are extremely powerful, and contract with 
great force, by means of a curiously spiral arrangement of the muscular fibres. These latter 
chambers are used for the purpose of propelling the blood through the body, while the auricles 
serve to receive the blood from the vessels, and to throw it into the ventricles when they are 
ready for it. 
By the systematic expansion and contraction of the heart-chambers, the blood is sent on 
its mission to all parts of the body, through vessels named arteries, gradually diminishing 
in diameter as they send forth their branches, until they terminate in branchlets scarcely 
so large as hairs, and which are therefore called “ capillaries,” from the Latin word capillus , 
a hair. 
In the capillaries the blood corpuscules would end their course, were they not met and wel- 
comed by a second set of capillaries. These vessels take up the wearied and weakened globules, 
carrying them off to the right-hand chambers of the heart, whence they are impelled through 
a vessel known by the name of the “ pulmonary artery,” to be refreshed by the air which is 
supplied to them in the beautiful structure known as the lungs. Meeting there with 
new vitality — if it may so be called — the blood corpuscules throw off some of their effete 
portions, and so, brightened and strengthened, are again sent through the arteries from the 
heart to run their round of existence, and again to be returned to the heart through the 
veins. 
It is indeed a marvellous system, this constant circular movement, that seems to be in- 
herent in the universe at large, as well as in the minute forms that inhabit a single orb. The 
planets roll through their appointed courses in the macrocosmal universe, as the blood globules 
through the veins of the microcosm, man : each has its individual life, while it is inseparably 
connected with its fellow-orbs, performing a special and yet a collective work in the vast body 
to which it belongs ; darkening and brightening in its alternate night and day until it has com- 
pleted its career. 
In order to prevent other organs from pressing on the heart, and so preventing it from 
playing freely, a membranous envelope, called from its office the “pericardium,” surrounds 
the heart and guards it. 
The various operations which are simultaneously conducted in our animal frame are so 
closely connected with each other that it is impossible to describe one of them without trench- 
ing upon the others. Thus, the system of the circulatory movement, by which the blood passes 
through the body, is intimately connected with the system of respiration, by which the blood 
is restored to the vigor needful for its many duties. 
In order to renew the worn-out blood, there must be some mode of carrying oif its effete 
particles, and of supplying the waste with fresh nourishment. For this purpose the air must 
be brought into connection with the blood without permitting its escape from the vessels in 
which it is confined. The mode by which this object is attained, in the Mammalia, is briefly 
as follows : — 
A large tube, appropriately and popularly called the “windpipe,” leads from the back of 
the month and nostrils into the interior of the breast. Just as it enters the chest it divides 
into two large branches, each of which subdivides into innumerable smaller branchlets, thus 
forming two large masses, or lobes. In these lobes, or lungs, as they are called, the air-bear- 
ing tubes become exceedingly small, until at last they are but capillaries which convey air 
instead of blood, each tube terminating in a minute cell. The diameter of these cells is very 
small, the average being about the hundred and fiftieth of an inch. Among these air-bearing 
capillaries the blood-bearing capillaries are so intermingled that the air and blood are separated 
from each other only by membranes so delicate that the comparatively coarse substance of the 
blood cannot pass through, although the more ethereal gases can do so. So, by the presence 
of the air, the blood is renewed in vigor, and returns to its bright florid red, which had been 
lost in its course through the body, while the useless parts are rejected, and gathered into the 
air-tubes, from whence they are expelled by the breath. 
The heart is placed between the two lobes of the lungs, and is in a manner embraced by 
them. The lungs themselves are enclosed in a delicate membrane called the “pleura.” These 
