PROFESSOR SIMONDS’ LECTURE. 
517 
atmosphere. It extended into the divisions of the tube, called the 
bronchi, and was lost in the cells of the lungs. This was not 
unfrequently the seat of disease. They had all, doubtless, heard 
of bronchitis, inflammation of the larynx, and common catarrh. 
He pointed out, by means of a drawing, the passage for the air 
from the mouth and nostrils to the lungs, until it terminated in 
what were called the air-cells. With respect to the lungs them¬ 
selves, he observed that they would require a very short descrip¬ 
tion. They might be termed light, spongy bodies filled with air- 
cells and vessels, and covered with pleura. A considerable quan¬ 
tity of elastic tissue entered into their composition, and it was also 
evident that a certain amount of contractile tissue existed. The 
respiratory motion which we perceived when a person breathed, 
was partly caused by the active operation of those powers, and 
was not, as had been said, altogether owing to the compression and 
expansion of the chest. In the act of expiration a portion of the 
air was forced out by compression of the sides, and the pressing 
forwards of the abdominal viscera, by the contraction of the 
muscles. This action ceasing, the ribs, aided by their cartilages, 
sprung back, and, the diaphragm contracting at the same time, the 
dimensions of the thorax were thereby increased. The air con¬ 
tained in the lungs by its rarefaction and elasticity caused them 
to expand, and a rush of fresh atmospheric air down the windpipe 
into the cells took place to equalize its density. This caused the 
motion which was felt in breathing. As the chief use of this 
function was to eject carbonic acid gas from the system, and to 
produce oxygenated blood, so the quantity of air respired was 
regulated accordingly. It was impossible accurately to convey 
an}' just idea of the quantity of air inhaled and exhaled at each 
respiration. The quantity of carbonic gas evolved varied from 
four to even eight per cent.; but the quantity was governed in a 
great degree by the age of the animal, and its exertions or mus¬ 
cular movements, for he ought to tell them that every movement 
was accompanied by a corresponding alteration in the condition of 
the tissues. As an animal underwent increased exertions, in 
proportion would be the increased quantity of carbon produced in 
the system; and therefore it was necessary that a proportionate 
quantity of oxygen should be inhaled by increased breathing, so as 
to counteract its effects, otherwise death would ensue. This was 
sometimes shewn to be the result of over-exertion in the case of 
horses. Hunters had been known to drop down and die in con¬ 
sequence of being unable to inhale a sufficient quantity of oxygen 
to unite with the carbon produced during their violent exertions. 
After dwelling a little longer upon this part of his subject, and 
illustrating the peculiarities of the organs of respiration in the ox 
and the horse by a reference to drawings, the Lecturer passed to the 
