Bauhaus-Universität Weimar

Dictionary of philosophy and psychology including many of the principal conceptions of ethics, logics, aesthetics ... and giving a terminology in English, French, German and Italian, vol. 2 [lead-zwing]
Baldwin, James Mark
(2nd ed.), 188; Saxeoed, Course in Exper. 
Psychol., expt. 110. (e.b.t.) 
Muscle [h&t.musculus, little mouse, dim. of 
mus, Gr. juOs] : Ger. Muskel ; Fr .muscle; Ital. 
muscolo. The active movements of animals 
are accomplished by means of a specially 
differentiated contractile tissue called muscle. 
Contractility is a general property of living- 
organisms manifested in amoeboid, ciliary, or 
muscular movements. Even in the unicellular 
Protozoa, certain portions of the cell-substance 
become specialized as contractile fibrils for 
executing movements, and in the lower 
Metazoa, such as the Coelentera (Polyps and 
Jelly-fish), cells of the outer and of the inner 
epithelium may develop special contractile 
basal processes. The muscular movements of 
the Coelomata are accomplished by means of 
special muscle-cells, or cross-striated muscle- 
fibres, developed from the mesodermal layer 
of the Embbyo (q.v.). Muscle-cells chiefly are 
found in Worms, Molluscs, and Echinoderms; 
striated muscle-fibres in Arthropods. In the 
higher vertebrates, such as man, the ‘ smooth ’ 
muscle-cells are found in the walls of the 
intestine and other viscera, and of the blood¬ 
vessels. Their contraction is not under the 
direct control of the will, hence they are 
frequently called the involuntary muscular 
tissue. These muscles are formed of elongated 
cells pointed at both ends, consisting chiefly 
of longitudinally striated contractile substance, 
enclosed in a delicate sheath, and provided 
with an oval nucleus. The muscles of the 
heart are composed of peculiar fibres made of 
rows of flattened, more or less branched, cells 
exhibiting transverse striations. The volun¬ 
tary or skeletal muscles, the contraction of 
which is under the direct control of the will, 
constitute by far the greater part of the 
muscular or fleshy portions of the body, and 
consist of cross-striated fibres. 
The fibres are formed of a thin sheath (sarco- 
lemma) enclosing the elongated, more or less 
cylindrical, soft muscular substance provided 
with numerous oval nuclei at its periphery. 
Under the microscope the muscular substance 
exhibits fine longitudinal and coarse transverse 
striations. The former seem to be due to the 
fact that each fibre is formed of a bundle of 
fine parallel fibrils (sarcostyles) ; whilst the 
cross striae are due to the alternation of narrow 
light with broader and more opaque regions. 
The denser and firmer substance of the sarco¬ 
styles appears to be the more actively con¬ 
tractile element, embedded in the more fluid 
substance (sarcoplasm) accumulated in the 
region of the light-zones. Contraction of the 
fibre is brought about by the shortening and 
swelling of the row of segments of the sarco¬ 
styles occupying the dark zones. Waves of 
contraction may pass along the fibre. 
Muscles are formed of bundles of such fibres 
bound together in connective tissue and extend¬ 
ing from one point of attachment to another. 
Movement of the parts is brought about by 
a drawing together of the points of attach¬ 
ment when the muscle contracts, portions of 
the bony skeleton often serving as levers. 
The energy required for the muscular con¬ 
tractions is derived partly from carbohydrates, 
such as glycogen, stored in the muscle itself 
or brought to it by the blood, and probably 
also from fats. 
Living muscle is very elastic and extensible. 
During contraction it shortens and becomes 
correspondingly thicker. On relaxation it 
reverts to its original shape by virtue of its 
elasticity. Normally, a muscle contracts on 
receiving a stimulus through its motor nerve ; 
but it may be made to contract by the appli¬ 
cation of suitable mechanical, electrical, ther¬ 
mal, or chemical stimuli, either directly or 
indirectly by means of its nerve. A single 
stimulus causes a single £ twitch ’ of the 
muscle ; the contraction is preceded by a very 
short 'latent period/ and is followed by a 
longer period of relaxation. On repeating 
the stimulus, the contractions at first slightly 
increase in strength, then begin to decrease, 
and steadily diminish, until finally the muscle 
reacts no more. This muscular ‘ fatigue ’ is 
due probably ‘ both to the accumulation of 
waste products and to the exhaustion of the 
materials which afford the source of energy. 
Within certain limits, the contraction of a 
muscle is proportional to the strength of the 
stimulus, being nil with less than minimal 
stimulus, and constant after the stimulus has 
reached its maximal intensity. A repetition 
of stimuli following each other so rapidly that 
the muscle has no time to relax, leads to the 
condition known as ‘ tetanus,’ in which the 
muscle remains in a state of contraction until 
the stimuli cease or it is exhausted. Persis¬ 
tent voluntary contractions are considered by 
some physiologists to be of the nature of 
a tetanus. 
Literature: E. A. Schäeee, Essentials of 
Histology, also in Quain’s Anat. (10th ed.); 
textbooks of physiology, e.g. Fosteb’s, Wal- 
leb’s, ‘American/ &c. For a full table of 
the human muscles, with figures, see Gould, 
Ulus. Diet, of Med., sub verbo. (e.s.g.) 


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