202 
RESPIRATION. 
the atmospheric air rashes through the nostrils, trachea, and 
bronchial tubes, and inflates the lungs. At the same instant the 
blood flows from the right side of the heart, through the pulmo- 
nary artery, to the air-cells ; the living principle is received into 
the circulating fluid ; and the pulmonary veins take up the vital 
current, and convey it onward to the left side of the heart, whence 
it is to proceed to serve all the purposes of the body. This is the 
first act in life — to inspire , or take air into the lungs. 
The before-named muscles entering into the formation of the 
walls of the thorax, contract and raise the ribs. The diaphragm 
by its contraction flattens, and thus the capacity of the chest is 
increased, and the subsequent effects of inspiration are fulfilled. 
This first and most important act of inspiration is immediately 
followed by an expulsion of air from the lungs, partly by their 
own elasticity contracting ; by relaxation of the inspiratory mus- 
cles allowing the walls of the chest to collapse ; by the pressure of 
the atmosphere externally and their own property of resuming their 
original position ; by relaxation of the muscular structure of the 
diaphragm, and by contraction of the muscles of expiration, viz. 
the abdominal external and internal oblique muscles and the 
transverse and recti muscles of the abdomen, which are all mus- 
cles of expiration. The most important of them are the external 
oblique, which muscles, by their contracting, force the abdominal 
viscera against the diaphragm and carry it upward and forward 
into the thorax, at the same time they also retract the ribs late- 
rally, diminishing the cavity of the chest from behind forward 
by protrusion of the diaphragm, and on each side by compression 
of the ribs : by these agents the air is returned from the lungs, 
and the term expiration is applied to this movement. The whole of 
the air, however, inspired at the first ingression immediately fol- 
lowing birth is never again wholly evacuated from the lungs, — a 
portion ever afterwards remains in the tubes. Even in the deepest 
and most prolonged expiration the lungs are never emptied of 
inspired air, as expiration carried beyond a certain extent, is met 
by increasing pressure of the atmosphere upon the larynx, and the 
moment that the powers of expiration are exhausted, a fresh volume 
of air enters the air-tubes, maintaining an equilibrium between 
the atmospheric pressure within the lungs and without the chest 
in beautiful counterpoise. These acts of respiration are princi- 
pally performed by the diaphragm in ordinary breathing. It is 
the most important muscle engaged in the respiratory movement. 
Its extent of muscular surface and attachment extending across 
and constituting the large oblique posterior boundary of the chest, 
sanction a precedence to this agent above all the others employed 
in the mechanism of respiration. The muscular fibres of thismus- 
