VOLUNTARY MUSCULAR TISSUE 419 



and lymphatics, with certain nuclear and cellular anatomical elements. 

 The muscular system in a well-proportioned man is equal to about two- 

 fifths of the weight of the body. Among the characteristic properties 

 of the muscles are elasticity, a constant and insensible tendency to 

 contraction, called tonicity, the power of contracting forcibly on the 

 reception of a proper stimulus, and a peculiar kind of sensibility. The 

 muscular tissue is made up of a great number of microscopic fibres, 

 known as primitive muscular fasciculi. These are called red, striated 

 or voluntary fibres. Their structure is complex, and they may be sub- 

 divided longitudinally into fibrillae and transversely into disks. In very 

 short muscles, some of the primitive fasciculi may run the entire length 

 of the muscle; but the fasciculi usually are 1.2 to 1.6 inch (30 to 40 

 millimeters) in length. The fasciculi, however, do not inosculate with 

 each other, but the end of one fasciculus is united longitudinally with 

 the end of another by a strongly adhesive substance, the line of union 

 being oblique ; so that the fibres practically run the entire length of the 

 muscle. Exceptions to this arrangement are the branching muscular 

 fibres of the heart and of the face and tongue. Each fasciculus in 

 ordinary muscular tissue is enclosed in its own sheath, without branching 

 or inosculation. This sheath contains the true muscular substance only, 

 and it is not penetrated by bloodvessels, nerves or lymphatics. The 

 color of the muscular fibres by transmitted light is a delicate amber, 

 resembling the color of the blood-corpuscles. 



The primitive fasciculi vary 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 physiological increase in the size of a muscle from exercise is due 

 to an increase in the size of the preexisting fasciculi and not to the forma- 

 tion of new elements. In young persons the fasciculi are yyVo to ysVo 

 of an inch ( 1 5 to 20 /-i) in diameter. In the adult they measure ? J 7 to 

 2^ of an inch (55 to 100 /x). 



The appearance of the primitive muscular fasciculi under the micro- 

 scope is characteristic. They present regular transverse striae, formed 

 of alternating dark and clear bands about ^^o~o ^ an * ncn C 1 A 1 ) wide. 

 With a high magnifying power, a fine transverse line is observed run- 

 ning through the middle of each one of the clear bands. In addition 

 they present longitudinal striae, not so distinct, and difficult to follow to 

 any extent in the length of the fasciculus, but tolerably well marked, 



