PHYSIOLOGY. 
instances of the same fluid secreted by or- 
gans of very different obvious structure. 
How, or why, certain organs secrete cer- 
tain liquors, is the most important and essen- 
tial question in this subject ; but one to which 
our ignorance will not enable us to reply 
in a satisfactory w'ay. Probably the chief 
and proximate cause consists in difference 
of structure, and perhaps in the arrange- 
ment of the minute vessels, which are the 
organs of secretion. The peculiar powers 
of each part, its share of irritability, and 
contractility, must also have an important 
influence. The rrrechanical explanation of 
the phenomenon, by the straiiring of the 
•fltrids through different sized pores, cannot 
be admitted for a moment. We have one 
flitid, the blood, sent into different organs ; 
Cilch of which separates from it a different 
produce of matter, differing in many in- 
stances from any contained before in the 
blood. Here then must be a decomposition 
and a recombination of elements produced 
by the living action of the gland. 
'Nuiniion may be considered as the com- 
pletion of the assimilating funclions ; to 
which the processes hitlierto described, un- 
der the heads of Digestion, Absorption, 
Circulation, Respiration, and Secretion, are 
only preliminary and preparatory. The 
food, changed in the manner we have al- 
ready described, aniraalised and rendered 
similar to the being which it is designed to 
nourish, applies itself to those organs whose 
losses it is to supply, and this identification 
of nutritive matter to our organs constitutes 
nutrition. The component parts of the 
living body are incessantly carried off by 
various causes. Thus tlie machine is con- 
tinually destroyed, and at distant periods of 
life does not contain any of its original 
elements. Madder, mixed witli the food, 
<lyes the bones of a red colour, which dis- 
appears when the use of the root is suspend- 
ed. These’ phenomena can only be explain- 
ed by admitting an entire removal and 
renewal of the bony particles. Now if the 
most compact and solid parts hg in a con- 
tinual motion of decomposition and recom- 
position, this motion must be more rapid 
•where the constituent principles arp in the 
smallest degree of cohesion, as in fluids. 
Physiologists have endeavoured to deter- 
mine the period of the entire renovation of 
tlje body, and have considered that an in- 
terval of seven years is necessary for the 
original particles to lie totally obliterated, 
and tlicir place supplied by others. 
When tlie nutritive matter has been duly 
assimilated, the parts which it supplies re- 
tain 'it, and incorporate it with their own 
substance. This nutritive appropriation is 
variously effected in different structures ; 
since each part converts to its own use, by 
a true secretion, tliat which is found analo- 
gous to its nature, and rejects the heteroge- 
neous particles. 
The mechanism of nutrition would be ex- 
plained if we could understand how each 
function divests the aliments on wdiich we 
exist, of their characters, to invest them 
with the properties of our organs ; how 
each individual part co-operates in changing 
their nutritious principle into onr own pecu- 
liar structure. Vegetables, which form tlie 
sole nourishment of man in many instances, 
and a very great share of it in all cases, con- 
sist chiefly of carbon, hydrogen, and oxygen, 
with sometimes a small quantify of azote and 
salts. In the organs of the man fed on 
these vegetables, azote predominates, and 
many new products are discovered, not dis- 
tinguishable in the aliment, and therefore 
formed in the act of nutrition. Every living 
body, without exception, possesses this 
faculty of forming and decomposing sub- 
stances, and of giving rise to new products. 
The marine plant, wjiose ashes form soda, 
if sown in a box filled with earth that does 
not contain a particle of that alkali, and 
moistened with distilled water, furnishes it 
in as great a quantity as if the plant had 
been growing on the borders of the sea, 
and always supplied with salt water. 
Living bodies tlien are elaboratories, in 
which such combinations and decomposi- 
tions occur, as art cannot imitate j bodies 
that to HS appear simple, as soda and silex, 
seem to form themselves of other parts ; 
while some, whose composition we cannot 
determine, as certain metals, suffer inevit- 
able decompositions; from which W'e may 
fairly conclude, that the powers of nature 
in the composition and decomposition of bo- 
dies far surpass the science of chemists. 
SENSATIONS. 
Vision. The mode in which the rays of light 
are affected, in passing through the various 
parts of the eye, is explained under the 
article Optics. See also Vision. We 
have only to add a few remarks on the phy- 
siology of the eye. 
The quantity of light that can enter the 
eye depends on tlie state of the pupil ; 
which is again influenced by the motions of 
the iris. When, after shutting the eye-lids, 
they are suddenly opened in a strong light, 
a disagreeable impression takes place on 
