THE PHYSIOLOGY OF THE CIRCULATION. 
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tained, although complicated, function ! On it depend nutri- 
tion, secretion, respiration, absorption, sensation, motion. What 
part could be formed, what fluid elaborated, what excrementi- 
tious matter removed, or what sensation exist, without a heart, 
arteries, and veins ? On the one hand, digestion would go on in 
vain ; on the other, absorption would be useless. For were 
there no arteries to carry the newly-formed blood to every part 
of the body, digestion could not support life ; so, if veins 
did not exist to receive the refuse matter from the absorbents, 
nutrition could not be perfected. How beautifully is every part 
of the frame deposited, when and where required ! Where bone 
is necessary, osseous matter is laid ; fleshy fibre, where muscle 
is called for; and cartilage, where neither bone nor muscle could 
be substituted. 
The simple mechanism of the circulation presents a most 
beautiful display of nature’s skill ; a mechanism, whereby tissues 
the most different are constructed out of the same fluid, and in 
which vessels externally alike elaborate secretions having scarcely 
one character in common. Who has discovered any difference in 
the blood going to the stomach and the kidneys ? Yet the fluids 
secreted by those organs are very dissimilar. Again ; the vessels 
supplying the brain, and those going to the teeth, are externally 
alike, and their contents are the same ; yet no two substances in 
the body differ more than medullary matter and enamel. What 
a display of Divine wisdom is seen in the timely deposition of 
each tissue, and that in its exact quantity ! 
That side of the heart which propels the blood through the 
system, and the arteries which convey it, are much stronger than 
the veins which return it and that side of the heart which merely 
propels it through the lungs. The arteries have three coats, an 
external elastic, a middle muscular, and an internal serous. The 
elastic is more developed near the heart, and the muscular less so ; 
but as they proceed, the elastic becomes less, and the muscular 
more. The advantage of this appears evident ; for as they are 
more distant from the heart, and consequently receiving less 
influence from its action, they require more power in themselves 
to keep up the regularity of the circulation. That they have this 
power has been proved by numerous experiments, especially 
where the heart has been removed. In the fish tribe, there is 
only a pulmonic heart to carry on the circulation. It is an 
interesting inquiry, but somewhat difficult of solution, what is 
the agency by which this machinery is wrought upon ; or what are 
the powers which are brought to bear upon it so as to put it into 
motion, and to keep up that motion without ceasing, without 
weariness, without thought, for the most part without conscious- 
