388 



ANIMAL MECHANICS. 



bones, muscles, and joints of animals may be explained by common 

 descent f rom a supposed common ancestor. 



Shoulder Joint. — The shoulder girdle differs from the 

 pelvic, in being moveable, so that the centre of the socket 

 round which the arm revolves is not fixed in one definite 

 position, but may be fixed for the moment by the action of its 

 own muscles, or it may be itself in motion during the motion 

 of the arm. Notwithstanding its greater complexity of action, 

 the same principles may be applied to the shoulder joint, 

 that were used in the discussion of the hip joint; and they 

 lead us to the same general conclusions, as to the absolute 

 perfection of the mechanism of the shoulder joint in all 

 animals. 



If we compare together the muscles of the shoulder and 

 hip, setting aside those of the shoulder which are peculiar to 

 it, from the mobility of the shoulder girdle, we find the fol- 

 lowing Table of comparative muscles, corresponding to each 

 other in action : — 



Hip Joint. 

 Muscles of Abduction. 

 Agitator caudse. 



Glutaeus primus. 



Shoulder Joint. 

 Muscles of Abduction. 



1. Trapezius inferior, . 



2. Trapezius superior, ] 



3. Deltoideus, 



4. Teres major, Tensor vaginae femoris. 



5. Supraspinatus, (xlutaius secundus. 



6. Infraspinatus, Glutseus tertius. 



7. Teres minor, Glutseus quartus. 



8. Infraspinatus secundus, 



9. Subclavius, 



Muscles of Flexion. 



Glutseus quintus. 

 Obturator internus. 

 Muscles of Flexion. 



Tr 



iceps 



lono-us, ..... Rectus femoris. 



2. Tricipiti accessorius, 



Sartorius. 



Subscapulars, Iliacus. 



Latissimus dorsi, 



Psoas magnus. 



