INSPIRATION. 
201 
backward towards the diaphragm, against which it comes in 
contact during expiration : it also inclines to the left side, so 
that the shock produced by its action is readily felt when the 
hand is applied to that side in the region of the organ. The 
heart contains four cavities, two on the right, and two on the 
left. The right side of the heart may be very properly termed the 
respiratory portion of the organ, considering the circulation as 
double, and the whole of the blood entering the right cavities 
passing from them to the lungs, through the pulmonary artery. 
This vessel arises anteriorly and superiorly from the base of the 
organ, and almost immediately divides into the right and left 
pulmonary trunks, each entering its respective lung, in com- 
pany with the bronchial tube, and, like the latter, divides and 
subdivides to extreme minuteness, ultimately anastomosing over 
the air-cells in a most delicate mesh-work of vessels whose dia- 
meters do not exceed of an inch, or the square of the mesh- 
like structure more than of an inch in width. In the same 
degree of diminutive capacity commence the pulmonary veins, by 
a union with the capillaries of the arteries, and uniting and re- 
uniting, thus increasing in size, pursuing their course towards the 
heart much in the same direction in which the arteries had left it, 
until, having arrived at the left auricle, they terminate in the su- 
perior posterior part of this cavity by four openings ; and here the 
pulmonary circulation may be said to cease. The branches of 
the bronchial tubes and air-cells and the trunks of the pulmonary 
arteries and those of the veins are supported on every side by a 
plentiful formation of cellular tissue ; the whole constituting, by an 
outward investment of the pleural membrane, the lobes of the lungs. 
Th is pleural membrane is a secreting surface, yielding a bland 
fluid, by which friction and adhesion are entirely prevented. 
Having given a concise description of the respiratory machine, 
I proceed to explain its movements or mechanism. Respiration, 
or breathing, is composed of two acts; 1st, that of inspiration, 
by which air is taken into the lungs; 2d, of expiration, by which 
it is expelled from them. In the foetal state the lungs are passive, 
the trachea and bronchial tubes are without atmospheric air in 
them, so that the first act of life in an animal is that of in- 
spiration. During the birth the umbilical cord is compressed, 
the circulation of the blood is obstructed, not only in the cord, 
but the whole circulation of the body of the foal is impeded. Upon 
delivery of the offspring the cord is ruptured. The pause thus 
given to the blood’s progress imparts an impulse to the nerves of 
the respiratory muscles, the fibres of which, previously relaxed 
and passive, suddenly and actively contract, the chest is en- 
larged and a vacuum begins to form in the cavity, to meet which 
