6 RESPIRATION. 
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 im its 
alternate night and day until it has completed its career. an 
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, 
swrounds 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 deseribe one of them 
without trenching 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 vigour needful for its many duties. 
In order to renew the worn-out blood, there must be some mode of carrying off its 
effete particles, and of supplying the waste with fresh nourishment. For this purpose 
the air must be brought into connexion with the blood without permitting its escape 
from the vessels in which it is confined. The mode by which this objeet is attained, in 
the Mammalia, is briefly as follows :— cS Wed AN 
A large tube, appropriately and popularly called the “windpipe,” leads from the 
bac’x of the mouth and nostrils into the imterior 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-bearing 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 vigour, 
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 accompanying illustrations 
will give a good idea of the capillary 
structure. Fig. I represents the air- 
tubes of the lungs, and fig. 2 ex- 
AT Te i re LUNGS. ieee THE LUNGS, hibits the capillaries through which 
the blood is conveyed. 
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 ealled 
the “pleura.” These two great vital organs are situated in the breast, and separated 
from the digestive and other systems by a partition, which is scientifically known by 
the name of “diaphragm,” and in popular language by the term “midriff” Tais 
structure does not exist in the Birds ; and its presence, together with that of the freely- 
suspended Jungs, is an unfailing characteristic of the Mammalian animal. ; 
Thus the entire structure bears the closest resemblance to a tree, growing with 
its root upwards and its leaves downward,—the trachea being the trunk, the branchial 
tubes the Fibs, the smaller tubes are the branches, and the air-cells the leaves: 
A similar idea runs through the nerve system and that of the blood; all three being 
interwoven with each other in a manner most marvellous and beautiful. 
;. 
a 
BY) 
Save 
AY 
