$4 THE HUMAN MOTOR 



69. (11) Muscles and their Resistance. Muscles - are -fleshy 

 masses of varying thickness, which are attached to the bones, 

 and partially retain them in their proper relative positions. 

 They are, properly speaking, the agents of movement, as will be 

 explained later. " 



Muscles are made of extremely thin parallel fibres, in a section 

 1 centimetre square, there may be nearly 100 millions of them. 

 They are enveloped by a transparent membrane, and terminated 

 by elastic laminae called " tendons," by which they are connected 

 to the skeleton. 



The fibres of the muscles of locomotion have a special con- 

 struction. Instead of 

 being simple elongated 

 cells, they are striated 

 transversely, presenting 

 the appearance of a pile of 

 discs, some transparent, 

 others opaque. These " 

 striated fibres are anis- 



tropic and their contents mimnil I || || I Illl IIH7/, 



appear to correspond to _ __ 'I ' 



the structure of colloids, (rjjp7IIIHIMI III II1J/D/ 'c 



along with some elements - . 



called micella which are t(V > 



FIG. 103. 



more stable than the 



gelatinous liquid which Scheme of striated muscular fibre with 

 2n T-, its nervous connexions, 



fills the fibres. The 



external aspect of a striated fibre may be compared with that 

 of a spiral spring enclosed in a glass cylinder ; the spirals would 

 project in parallel striae (fig. 103). Striation is a dynamic state 

 acquired by the protoplasma (*). 



Nervous filaments of great tenuity approach the muscular 

 fibres and come into contact with them, directly or indirectly 

 the terminal arborization C distributing itself to the muscular 

 elements and governing them, but it is not known exactly how. 

 The composition of muscles is 74% water, on an average in the 

 adult man : there is more in women and children. Their density 

 exceeds that of water by 75, because the liquid or muscular 

 plasma contains salts and albuminoid bodies, etc. The density 

 is slightly less in the female sex. 



It is obvious that the more fibres there are in a muscle, the 

 greater is its resistance ; i.e. the resistance is proportional to the 

 mass of the muscle. It must be remembered that the muscles 



( l ) Vies, Proprietes optiques des muscles. These, Paris, 1912. 



