GENERAL CONSIDERATIONS ON THE STRIPED MUSCLES. 



237 



Fig. 154. 



itself after being inflated, aiid that the transvei-se striation becomes much closer ; 

 though the zigzag doubling mentioned by Prevost and Dumas has not been 

 observed. 



But these physical and chemical modifications, important as they are from a 

 physiological point of view, cannot longer be dwelt upon here. It is particularly 

 important to speak of muscular contraction. 



A nuiscle that contracts becomes shortened ; its two extremities approach each 

 other if they are free ; or one draws near 

 the other if the latter is fixed to an im- 

 movable point. If the extremities of a 

 muscle are attached to two movable 

 Ievere,its contraction will bring about the 

 displacement of one or other of these ; 

 from this a movement is produced. 



The degree of shortening of a muscle 

 varies, according to its being entirely 

 free, or having a resistance to overcome. 

 The mean limit of tliis shortening is 

 about one-fourth the length of the 

 muscular fibres. From this it will be 

 understood, that the movement produced 

 by the contraction will be in proportion 

 to the length of the fibres ; though in 

 this appreciation it will be necessary to 

 keep in mind the density and energy of 

 the fibre, as well as the intensity of the 

 stimulation which induces the con- 

 traction. 



PORTION OF AN ELEMENTARY MUSCtJLAR FIBRE, 

 WITH FOUR DARK-BORDERED FIBRES (o) 

 CROSSING ITS SURFACE. 



b, Capillary blood-vessel, with fine nerve-fibres 

 (a few only of the transverse markings of the 

 muscle are represented); c, two of the dark- 

 bordered nerve-fibres passing over the ele- 

 mentary fibre to be distributed to adjacent 

 fibres. This arrangement, in which a dark- 

 bordered nerve-fibre, distributed to muscle, 

 divides into branches, one of which passes to 

 a vessel, while the other ramifies upon a 

 muscle, is frequent. Magnified 700 diameters. 



As each fibre represents a force 

 independent in its action, it results that the power of a muscle may be inferred by 

 the number of its fibres, or its volume. 



Muscles are often aided in their action by mechanical conditions : such as the 

 disposition of the levers on which they act, the direction of the muscular fibres 

 in connection with these levers, and, lastly, by the presence of bands or elastic 

 cords. 



C. Uses of Muscles. — There are ffexor, extensor, abductor, adductor, rotator^ 



Fig. 155. 



MUSCULAR FIBRF, IN A STATE OF CONTRACTION IN THE CENTRE : THE STRI^ APPROXIMATED, 

 THE BREADTH OF THE FIBRE INCREASED, AND THE MYOLEMMA RAISED IN VESICLES ON ITS 

 SURFACE. 



and other muscles, for all the movements of which the articulations are the 



centre. 



To determine the uses of the muscles, it is sufficient to know their insertions 



and the mode in which the bones furnishing these insertions articulate with each 



other. 



18 



