604 MR. youatt's veterinary "lectures. 
ference to others, I shall thus proceed, and shall reserve the sub¬ 
jects of our last course for the conclusion of the present one. 
I will commence, then, by inquiring into the change which 
takes place in the blood during the shorter or pulmonary circula¬ 
tion: or, in other words, the respiratory system will first 
occupy our attention during the present course. 
Respiration is the act of breathing—the alternate reception and 
expulsion of air into and from the chest. Our subject, then, ad¬ 
mits of two natural divisions—the mechanism by which the air is 
inhaled and forced out, and the effect produced during its con¬ 
tinuance in the chest. 
The thorax or chest contains, beside the heart and thymus 
gland, two large spongy bodies, and by these substances it is 
completely filled. When we trace the minute structure of these 
spongy bodies, the lungs, we find that they consist of nu¬ 
merous bloodvessels designed to convey nourishment to them, 
and of a far greater number of others bringing blood from the 
heart, and returning it to that organ ; and also thousands of other 
little tubes, carrying no blood, but only air—traced in one direc¬ 
tion to blind pouches or bags, and in the other, through the 
trachea, and pharynx, and mouth or nose, to communication with 
the atmospheric air; and all surrounded by and embedded in a 
great quantity of elastic cellular substance called the parenchyma, 
capable of expansion or compression, and yielding to the slightest 
impulse or power. 
The thorax is bounded anteriorly by the vessels which enter 
it or proceed from it, and the cellular substance by which they 
are surrounded; posteriorly, by the diaphragm; laterally, by the 
ribs; superiorly, by the spine; and inferiorly, by the sternum: 
by means of certain muscles and cartilages it is capable of en¬ 
larging or contracting its dimensions to a very considerable 
degree. 
The diaphragm is a strong muscular curtain, partly fleshy and 
partly tendinous, stretched in a slanting direction from the spine 
to the sternum, and dividing the chest from the abdomen. In its 
natural state, it projects into the chest. It is convex towards the 
chest, and concave towards the abdomen; and it is supplied with 
nerves from the lateral or respiratory column. 
Stimulated by the phrenic nerve, it contracts. The action of 
every muscle is that of contraction. As it contracts it must become 
straighter; and its bellying or convexity towards the chest must 
be lessened in proportion to the power applied to it. And what 
is the consequence of this contraction ? That the cavity of the 
chest will be in the same proportion enlarged. Then, as in the 
