70 STRUCTURE OF MUSCLE. 
to keep them in working order. For every development of 
animal force involves a change of state of the Nervo-mus- 
cular substance: a certain amount of it ceasing to exist as 
living tissue, and passing into the condition of dead matter ; 
and its elements resolving themselves, under the influence of 
the free oxygen brought to them by the blood, into new combi- 
nations, which are carried forth from the body as quickly as 
possible. Consequently, if the Nervo-muscular tissues be not 
renewed as rapidly as they are used up, their powers must 
speedily fail from the progressive loss of their substance. In 
this particular they are on a different footing from the other 
elementary parts of the organism ; for although each of these 
seems to have a certain term of life, the length of which is — 
in some degree related inversely to its functional activity,— 
those which live the fastest having the. shortest individual 
duration, and vice versé,—there are none which are called 
upon to give forth their whole vital energy in one effort, and 
which may thus have their existence as parts of the living 
organism terminated at any moment by a demand for their , 
peculiar power. 
56. Muscular Fibre presents itself under two forms, which 
are ordinarily very distinct from each other; although it is 
probable that they may ultimately prove to be but modifi- 
cations of one and the same. ‘The first, which is known as 
the striated fibre, is that of which all those muscles are com- 
posed, which constitute what is commonly designated as 
“flesh” or the “lean” of meat. If any “joint” of meat 
be even cursorily examined, it will be seen that its whole 
substance is made up of distinct masses, held loosely together 
by areolar tissue ; and these masses, which are known as 
“muscles,” are easily isolated from each other by dissection. 
Every such Muscle is formed by the union of a number of 
bundles, having a generally parallel arrangement, which are 
closely bound together by areolar tissue, and are themselves — 
composed of bundles still more minute, united in a similar 
manner. These, again, may be separated in the same way ; 
and at last we come to the primitive fibres of which this 
tissue is composed. Each of these primitive fibres termi- 
nates at either extremity in tendinous fibre, which unites — 
with other fibres to form the tendinous cords or bands, that 
are attached to the points of the skeleton which the muscle — 
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