516 
PROFESSOR SIMONDS’ LECTURE. 
suspended for a short period. If we held the breath, we were 
compelled very shortly to resume the act of breathing, a fact which 
depended partly upon, and consequently demonstrated that there 
was, an accumulated quantity of carbonic acid gas in the system 
which it was necessary for the preservation of life we should ex¬ 
hale. The central organs of respiration were called the lungs, and 
Avere placed in a cavity which was described as the middle one of 
the frame. There were three great cavities formed by the skeleton 
of the animal—the first, or cerebral, which contained the brain and 
the nerves of sense; the second, or thoracic, Avhich contained the 
lungs and the heart, the chief organs of respiration and circula¬ 
tion; and the third, or pelvic cavity, which in females contained 
the organs of generation. The thorax or chest, although formed by 
the bones, had these arranged in a different manner from the other 
two cavities, it being necessary, in order to allow of the expansion 
of the lungs in respiration, that the bones of this cavity should give 
way, and therefore they found that the ribs were united to the 
trunk by means of moveable joints and elastic cartilages. In this 
was found a beautiful provision of nature, for although the bones 
were required to protect the cavity from injury, they were made 
sufficiently moveable to allow of the expansion of the lungs in the 
act of respiration. 
After explaining the action of the lungs and some of the other 
organs of respiration, he observed, that we might suppose these 
structures had no other part to perform, but that, if we closely 
examined them, we should find that they discharged a double 
office. The windpipe conveyed the air to the lungs. The upper 
part of the windpipe was called the larynx, Avhich presented the 
same general appearance in all animals, although it was modified 
in order to suit the tones uttered by each. It was composed of a 
number of cartilages; and if they directed their attention to the 
different sounds which were expressed by different animals—the 
braying of the ass, the neighing of the horse, the bellowing of the 
ox, the bleating of sheep, the barking of the dog, and the grunting 
of the pig—they would at once perceive the necessity which existed 
for the variety of formation that was observable in the construction 
of the larynx of these animals. The larynx was the organ of 
voice, and received the air as it was about to enter the windpipe, 
and also the air expelled from the windpipe, before passing from 
the mouth and nose. The larynx, then, communicated with the 
windpipe, which was likewise composed of cartilages, arranged in 
the form of rings. Cartilage was pliable and tough, not liable to 
be put out of order by natural means, and was, consequently, 
often used in the place of bone. The interior surface of the 
windpipe was lined with what was called a mucous membrane, the 
object of which was to shield the windpipe from the action oi the 
