Bauhaus-Universität Weimar

vital phenomena. Omitting in our enumeration those parts which 
receive from the nutritive force merely physical properties, the en¬ 
dowments of the other principal systems of the animal body may be 
indicated as follows: — 
1. Organs ivhich change the chemical composition of the fluids 
for the purposes of the general system; such are the secreting organs, 
the blood-vessels and lymphatics, and the lungs. The peculiar func¬ 
tion performed by these organs is not nutrition, for this is performed 
in all the organs of the body, but the change of the organic combina¬ 
tion of the elements in fluids which are in contact with them, by the 
influence of organic affinity. 
2. Muscular organs, which contract when acted upon by certain 
influences, their fibres becoming flexed in a zigzag form towards the 
spot where a change of their substance is produced, and thus short¬ 
ened. Haller has named the property possessed by muscles, of con¬ 
tracting under the influence of mechanical, chemical, and electric 
stimuli, irritability ; and the irritability of Haller can be ascribed 
to no other than muscular parts, while other structures are charac¬ 
terised by the phenomena of a different kind of excitability. By 
some writers this term of irritability has been greatly misapplied; 
thus they have spoken of an irritability in the nerves, as if at one 
time their irritability, at another their sensibility, could undergo a 
change. In the living body the action of the muscles is always de¬ 
termined by their nerves; and every cause which changes the com¬ 
position of the nerves, although but slightly, produces a discharge, 
as it were, of the nervous force; and, as the result of this, a contrac¬ 
tion of the muscles. Hence the study of muscular motions, and of 
spasmodic and paralytic affections generally, leads to the investiga¬ 
tion of the laws which regulate the action of the nerves. Motion 
accompanies all changes of composition; it takes place in the pro¬ 
cesses of formation, nutrition, and secretion, and an organic affinity 
exerted between the blood and the tissues produces the motions 
which accompany turgescence or erection. Muscles are not the 
only parts capable of motion, but they are the only organs which 
move by contraction and zigzag flexure of fibres; and all parts 
which are able to contract in this manner, although not essentially 
muscles, derive this power from muscular substance, particularly 
muscular fibres, intermixed with their tissue; such parts are the 
efferent ducts of glands, which are distinctly contractile. The tissue 
which gives the dartos its power of motion may be supposed to 
contract by the, inflection of its fibres, though of this we have no 
proof; but, after all, we may doubt that muscular fibres in all cases 
shorten themselves by zigzag inflection.* 
3. The nerves are in part motor, in part sensitive. The motor 
nerves are those which, under the influence of changes in their con¬ 
dition so slight as to elude the perception of the observer, excite 
motions in the muscles. The sensitive nerves are those which have 
the faculty of communicating every change of condition which they 
* See the Chapter on the Muscular and Allied Motions. 


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