528 MOVEMENTS. 



The ordinary length of these fibres is about ^fo, and their breadth about ^fa of an inch. 

 In the gravid uterus they undergo remarkable hypertrophy, measuring here from ^ 

 to ^ of an inch in length, and ^Vo- of an inch in breadth. The peculiarities of their 

 structure in the uterus will be fully considered under the head of generation. 



In the contractile sheets formed of involuntary muscular tissue, the fibres are arranged 

 side by side, are closely adherent, and their extremities are, as it were, dove-tailed into each 

 other. Generally the borders of the fibres are regular and their extremities are simple ; 

 but sometimes the ends are forked, and the borders present one or more little projections. 

 It is very seldom that we see the fibres in a single layer, except in the very smallest arte- 

 rioles. Usually the layers are multiple, being superimposed in regular order. The action 

 of acetic acid is to render the fibres pale, so that their outlines become almost indistin- 

 guishable, and to bring out the nuclei more strongly. If we have an indistinct sheet of 

 this tissue in the field of view, the addition of acetic acid, by bringing out the long, nar- 

 row, and curved nuclei arranged in regular order, and by rendering the fibrous and other 

 structures more transparent, will often enable us to recognize its character. 



Contraction of the Involuntary Muscular Tissue. The mode of contraction of the 

 involuntary muscles is peculiar. It does not take place immediately upon the reception 

 of a stimulus, applied either directly or through the nerves, but it is gradual, enduring for 

 a time and then followed by slow and gradual relaxation. A description of the peristaltic 

 movements 'of the intestines gives a perfect idea of the mode of contraction of these 

 fibres, with the gradual propagation of the stimulus along the alimentary canal, as the 

 food makes its impression upon the mucous membrane. An equally striking illustration 

 is afforded by labor-pains. These are due to the muscular contractions of the uterus, 

 and they last from a few seconds to one or two minutes. Their gradual access, continua- 

 tion for a certain period, and gradual disappearance coincide exactly with the history of 

 the contractions of the involuntary muscular fibres. 



The contraction of the involuntary muscular tissue is slow, and the fibres return 

 slowly to a condition of repose. The movements are always involuntary. Peristaltic 

 action is the rule, and the contraction takes place progressively and without oscillations. 

 Contractility persists for a long time after death. Arrest of function is followed by little 

 or no atrophy, and hypertrophy is very marked as the result of exaggerated action. Ex- 

 citation of the nerves has less influence upon contraction of these fibres than direct exci- 

 tation of the muscles. The involuntary muscular tissue is regenerated very rapidly, while 

 the structure of the voluntary muscles is restored with great difficulty after destruction 

 or division. (Legros and Onimus.) 



Physiological Anatomy of the Voluntary Muscles. A voluntary muscle is the most 

 highly organized and is possessed of the most varied endowments of all living structures. 

 It contains, in addition to its own peculiar contractile substance, fibres of inelastic and 

 elastic tissue, adipose tissue, numerous blood-vessels, nerves, and lymphatics, with certain 

 nuclear and cellular anatomical elements. The muscular system constitutes by far the 

 greatest part of the organism, and its nutrition consumes a large proportion of the repara- 

 tive material of the blood, while its disassimilation furnishes a corresponding quantity of 

 excrementitious matter. The condition of the muscular system, indeed, is 'an almost un- 

 failing evidence of the general state of the body, allowing, of course, for peculiarities in 

 different individuals. Among the characteristic properties of the muscles are, elasticity, 

 a constant and insensible tendency to contraction, called tonicity, the power of contract- 

 ing forcibly on the reception of a proper stimulus, called irritability, a peculiar kind of 

 sensibility, and the faculty of generating galvanic currents. The relations of particular 

 muscles, as taught by descriptive anatomy, involve special functions ; but the most inter- 

 esting physiological points connected with this system relate to the general properties 

 and functions of the muscles, and must necessarily be prefaced with a sketch of their 

 general anatomy. 



