2 
The change then lias been from a dark coloured 
fluid — two or three wares of which circulating 
through the brain would produce asphyxia and 
death in a few minutes — to a bright red fluid 
circulating through every part of the body, and 
carrying the elements of increase, health, and 
strength, to every organ. How has this change 
been effected ? 
The bronchial tubes, which commence oppo- 
site the third dorsal vertebra by the bifurcat ion of 
the trachea and enter each lung at its root — 
divide into two branches, and each of these 
divide and sub-divide dichotomous) y to their 
ultimate termination in the intercellular pas- 
sages and air cells. We have seen already 
that the capillaries of the pulmonary artery and 
pulmonary veins are continuous on the parietes 
of these cellular passages and air cells, and 
throhgli these parietes these changes in the 
blood and air take place. 
In tracing the bronchial tubes back to the 
point where they unite and form, we need only 
mention that this tube is composed of fibro 
cartilaginous rings, which, by their elasticity 
prevent its closure. Between it, however, and the 
base of the tongue is a complex and beautiful 
organ, the larynx, which must receive a pas- 
sing notice, as its closure by the epiglottis has 
carefully to be guarded against, in endeavour- 
ing to restore persons by the ready method. 
The larynx has been described as a shot tube, 
having an hour-glass form — the upper and 
more prominent part — the thyroid cartilage 
(like a shield), forms in front the pomum adarni. 
The lower, the cricoid cartilage (like a ring), 
narrow in front ahd broad behind — within are 
the organs of sound — the chordae vocales. The 
aperture of the larynx is a triangular opening, 
closed duiing deglutition by the epiglottis 
during which act the larynx is drawn up under 
the base of the tougue, and the particle of food 
passes over it into the pharynx. The violent 
spasmodic action, with which any substance is 
thrown out that may by accident get into the 
larynx, shows how carefully nature guards this 
important, complicated, and mod beautiful 
organ. 
We think that sufficient lias now been said 
of the organs concerned in respiration. We 
next consider briefly the chemical changes pro- 
duced by the act. “ The blood comes to the 
lungs charged with carbonic acid, formed by 
the union of carbon and oxygen in the 
systemic capillaries ; this, it imparts to the 
inspired air, at the same time abstracting from 
it a volume of oxygen, which is always as large, 
and usually greater. It appears probable that 
a part of this oxygen is made to combine 
with hydrogen set free in the systemic 
capillaries, and that the water thus 
generated forms part of that exhaled from the 
lungs. A sort of combustion of hydro-carbon 
thus appears to be continually going on in the 
body at large, the products of which are got 
rid of by the lungs ; and this process is 
mainly, if not solely, instrumental in the main- 
tenance of animal heat. The quantity of car- 
bonic acid excreted by the lungs has been esti- 
mated by some experimenters at as much as 
39,000 cubic inches in twenty-four hours. This 
amount of gas would contain 5148 grs , or 
11 ounces (troy), of solid carbon. This, how- 
ever, is considered too high : other very careful 
experiments gave 2616 grs,, or 5J ounces of 
solid carbon, in the same time. Probably the 
mean of these, or about 8 ounces, is the true 
quantity. 
Atmospheric air is a mixture (not a chemical 
compound) of 21 parts, by measure, of oxygen, 
a id 79 of nitrogen, or azote, and we trace from 
3 to 6 measures in 10,000 of carbonic acid. 
Air with 10 per cent, of carbonic acid will de- 
troy life. Probably 1 per cent, would do so, 
slowly. Where, then, does all the carbonic 
acid of expired air go ? It is taken up by the 
vegetable world — carbonic acid and water being 
the main food of plants, and by them the air 
is purified and made fit for re-inspiration; for 
though plants in their respiration, like animals, 
abstract oxygen from the air, yet the quantity 
is small, ani much more than compensated for 
by the large quantity of carbonic acid they con- 
sume for food. 
The changes in- the blood of the colour, from 
the dark purple of the venous to the rich crimson 
of the arterial ; of temperature, arterial blood 
being 2 deg. warmer; of density, the sp. gravity 
of arterial blood being lower ; and the altered 
relative quantity of oxygen and carbonic acid 
in the two— these are the chief changes. The 
red particles of the blood are supposed to be 
the oxygen carriers, and though the exact differ- 
ence between the two fluids is not, perhaps, 
known, yet we know that the oxygen is re- 
quired for the formation of the new tissues, etc., 
as well as by its union with hydrogen — to form 
animal heat — as we have already seen, and that 
carbonic acid is a result of broken down and 
worn out tissue, poisonous to the system, and 
eliminated from ic by the act of respiration — to 
be taken out of the air as the food of plants. 
Thus, by an interchange of the tlireo gasses, 
oxygen, carbonic acid, and hydrogen, the 
animal and vegetable world is wonderfully and 
beautifully secured. 
Next, we had purposed to take under review 
the nervous influence concerned in respiration, 
the pneumogastric — that wonderful compound 
nerve— the fifth (trifacial) ; these are the chief 
excitor nerves. Then the motor nerves, the re- 
spiratory nerves of Sir Charles Bell ; the 
phrenic, or int. respiratory ; the long thoracic, 
or ext. respiratory ; and the intercostals — these, 
with'the facial and spinal accessory also motor 
nerves connected with the act of respiration — 
we had intended to have described, from their 
origin in the medulla oblongata, or lower 
down from the upper part of the spinal cord 
to their distribution internally and externally 
to all the parts concerned in respiration ; then 
endeavoured to show their functions separately 
and in concert ; the effect produced by their 
