STRUCTURE OF THE VOLUNTARY MUSCLES. 



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It has been demonstrated by minute dissection that all of the red, or voluntary mus- 

 cles are made up of a great number of microscopic fibres, known as the primitive muscular 

 fasciculi. These are called red, striated, or voluntary fibres, or the fibres of animal life. 

 Their structure is complex, and they may be subdivided longitudinally into fibrillae, and 

 transversely into disks, so that it is somewhat doubtful as to what is, strictly speaking, 

 the ultimate anatomical element of the muscular tissue. 



A primitive muscular fasciculus runs the entire length of the muscle, and is enclosed 

 in its own sheath, without branching or inosculation. This sheath contains the true 

 muscular substance only, and it is not penetrated by blood-vessels, nerves, or lymphatics. 

 If we examine with the microscope a thin, transverse section of a muscle, the divided ends 

 of the fibres will present an irregularly polygonal form, with rounded corners. They 

 seem to be cylindrical, however, when viewed in their length and isolated. Their color 

 by transmitted light is a delicate amber, resembling somewhat the color of the blood- 

 corpuscles. 



FIG. 154. Striated muscular fibres, from the mouse ; magnified 500 diameters. (From a photograph taken at 



the United States Army Medical Museum.) 

 The injected capillaries are seen, somewhat out of focus. 



The primitive fasciculi vary very much in size in different individuals, in the same 

 individual under different conditions, and in different muscles. As a rule, they are smaller 

 in young persons and in females than in adult males. They are comparatively small in 

 persons of slight muscular development. In persons of great muscular vigor, or when 

 the general muscular system or particular muscles have been increased in size and power 

 by exercise, the fasciculi are relatively larger. It is probable that the physiological 

 increase in the size of a muscle from exercise is due to an increase in the size of the pre- 

 existing fasciculi, and not to the formation of any new elements. In young persons, the 

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