606 MR. youatt’s veterinary lectures. 
ration. In cases of urgent necessity we can press into the service 
every muscle of the chest and of the loins. 
What is effected by the rising and falling of these living bel¬ 
lows ? I will not detain you by enlarging on any of the secondary 
effects of respiration ; as the dependence of the voice on the pas¬ 
sage of air over or through the vocal organs; the occasional pres¬ 
sure on the thoracic and abdominal viscera accelerating or retard¬ 
ing the circulation, influencing digestion, and hastening the ex¬ 
pulsion of the fseces; the maintenance of the proper degree of 
temperature in the frame ; the sense of smell, for odoriferous par¬ 
ticles from surrounding objects can be received into the nostril 
only during the inhalation of air: but there is one fact,—the 
blood that flows from the left ventricle of the heart through the 
aorta is red, vital, able to support the secretions, and to nourish 
and build up the frame; but having thus parted with some 
of its substance and properties, or by the absorbing power of the 
vessels, or the direct influx of fluid through the absorbent, being 
mixed with other ingredients, and having acquired a different 
character, it becomes black, and, if not poisonous, yet inefficient; 
and no longer able to bestow on the tissues the power of contrac¬ 
tility. Supplied with venous blood alone, every muscle of or¬ 
ganic and of animal life w 7 ould cease to act; and that muscle, the 
heart, the very source of circulation, w^ould first stop, and the ani¬ 
mal would immediately die. This, then, is the grand effect pro¬ 
duced by respiration, to change the venous blood to arterial—the 
inefficient to efficient—to supply that without which no tissue 
will respond to the stimulus of the nerves either of animal or 
organic life, and without which no creature could live for an in¬ 
stant. 
How is all this effected ? What mysterious change is it that 
takes place in the circulatory fluid ? The venous blood is sent 
from the right ventricle of the heart to the lungs, and the pulmo¬ 
nary artery branches into divisions more and more minute, until 
they are many times less than a hair: and these minute vessels 
ramify over the cells in which the air-passages terminate. When 
these cells are distended with air, the air and the blood are se¬ 
parated from each other only by the thin parietes of the vessel, 
and the thinner membrane of the cell, and these delicate gossamer 
membranes are pervious to the air and to those component parts 
of the blood which are capable of assuming a gaseous form. 
It will be my pleasing task, in other lectures, to describe at 
length, and to illustrate by experiment, the chemical effect which 
now takes place. The blood has received, through the means of 
the various absorbents, or the chyle conveyed to it by the thoracic 
duct, a quantity of carbon, and it has other matters which require 
