ANIMAL MECHANICS. 



363 



joint which shall produce the maximum effect of all the 

 muscles. 



Muscles used in Abduction. — The whole group of muscles 

 (i to 8) whose action is abduction of the femur, either 

 wholly or in part, may be regarded as examples of muscles 

 of the same class as the great pectoral in the bird's wing ; 

 that is, in the position of maximum extension of the fibres, 

 the origin and insertion may be regarded as lying in the 

 same plane, which plane contains also the centre of the soc- 

 ket of the hip joint. Hence the position of this centre, for 

 maximum work, must lie somewhere on the axis of maximum 

 instability. 



i. Agitator caudce. — This muscle takes its origin, for a 

 length of 3.14 in. from the ist,.2nd, and part of the 3rd cau- 

 dal vertebrae, and is inserted by means of a long tendon into 

 the outer side of the patella. Its form is shown in Fig. 91, 

 in which AB is the 

 origin and A'B' the 

 insertion. S is the ac- 

 tual socket of the hip 

 joint, and 8T the axis 

 of rotation. The mus- 

 cle is so nearly trian- 

 gular in shape that we 

 may assume as a close 

 approximation (pp. 

 271, 289), that the 

 axis LM, which bi- 

 sects the intercept of 

 OX between the bones, 

 is the axis of maxi- 

 mum work. On mea- 

 suring the space be- 

 tween LM and ST, 1 

 found — 





j 







A ,000 









s\ 







T 





/ 



L \ 







M 





a'V 



J B 





0 



Fig. 91. 



