100 



CONTRACTILE ELEMENTS. 



times bony, and sometimes tendinous ; while, what is tendon 

 in man becomes bone in certain animals. In reptiles, for 

 example, the linea alba becomes a bone, and the intersections 

 of the recti muscles are represented by as many distinct 

 bones. In birds the tendons are represented in certain parts 



by bony stems, placed along the 

 extended portions of the principal 

 bones. The existence and length 

 of the tendons depend on the nat- 

 ure and extent of the motion to be 

 produced. Whenever this is of 

 great extent, and requires great 

 strength, muscular tissue is pre- 

 dominant throughout the muscular 

 apparatus, and is directly inserted 

 into the bone. Wherever the 

 movements of the bony parts are 

 of small extent, and require only 

 a slight shortening of the muscle 

 to produce them, we find the 

 / fibres of this muscle short, and 

 (jl ending in a true tendon. 

 N The force of a muscle is usually 

 estimated by the number of its 

 fibres, that is to say, by its thick- 

 ness and by its diameter (see p. 

 87). The length of a muscle, on 

 the contrary, corresponds with the 

 degree of displacement of the bones 

 (compare the sartorius and the 

 muscles of the ball of the thumb). We find some short 

 muscles, placed at great distances, and yet, compared with 

 one another, possessing slight degree of movement. In such 

 a case as this, a tendon takes the place of a large part of the 

 muscle, as in the case of numerous muscles of the fore- 

 arm, in which the muscular parts are short, and the ten- 

 dons very long : a greater length of muscular fibre would be 

 unnecessary here to produce such a slight displacement as 

 the flexion of the hand upon the forearm, and of the pha- 

 langes against each other. The extensor carpi ulnaris 

 muscle appears to be an exception s .to Jthis rule; but, in 

 reality, although its fleshy >)art occupier, tho whole length of 



* ^ifi 9i h, Embn r on5c globujes of connective tissue.' 'Rtlati'on of these ele- 

 ments (pla55ii>itflc^ta'tibrus ' 



Fig. 26 * 



