LOCOMOTOR SYSTEM 603 



kept down no horse can rise. All ruminants rise tail foremost. 

 Resting on both knees, they raise the hind-quarters, straighten 

 the limbs, and then rise in front. 



Joints. — Before examining the second function of muscles — 

 viz., the production of locomotion — it is necessary to look again 

 at the question of joints. These have been studied in connec- 

 tion with their static function ; they must now be looked at 

 from their kinetic aspect. 



Whenever two bones come together a joint is formed and 

 arrangement provided against the production of friction. When- 

 ever any two surfaces meet, one of which is moving on the other — 

 such as tendons passing through grooves or working over bony 

 projections — similar anti-friction appliances exist. We have 

 even ventured to regard the adjustment of the fore-leg on 

 the chest wall as a joint, although wholly muscular, and have 

 pointed out (p. 590) in what way friction is provided against. 

 It has been shown that the joints of the limbs of the horse are 

 furnished with automatic appliances for maintaining position 

 with the least muscular effort, and enabling the animal to stand 

 continuously for long periods. The features now to be con- 

 sidered are those connected with the movements of joints. 



The Shoulder-Joint is remarkable for the small surface afforded 

 by the scapula, and the large area existing on the humerus. 

 This allows great freedom of movement in both a to-and-fro 

 and side-to-side direction. There is no bone in the body the 

 movements of which resemble the scapula, and there is no 

 joint which resembles that formed by it with the humerus. 

 There are no separate ligaments to the joint ; the tendinous 

 insertions of the muscles are the ligaments, and this provision 

 would appear to be associated with the immense freedom required 

 in the rolling action of one bone on the other. Both in the fore 

 and hind limbs the two upper bones of the leg are arranged to form 

 an obtuse angle, and this is peculiar to quadrupeds. In the 

 biped the humerus is not bent from the vertical, nor are the 

 femur and tibia. The object of the bending is to ensure great 

 rapidity of action and destroy concussion ; there is far less jar 

 in dropping on a limb in which the column of bones is bent 

 towards each other than in a column which is vertical. The scapula 

 and humerus form an angle of about 120 degrees. The shoulder- 

 joint is lifted — viz., the fore-limb is advanced- — by only one muscle, 

 the mastoido-humeralis [levator humeri) (Fig. 187). Arising from 

 as high as the head and atlas, this immense muscle is solely 

 responsible for advancing the limb. The numerous and complex 

 movements occurring throughout the whole length of the leg 

 on the shoulder being raised may here be outlined. The shoulder 

 cannot be advanced with the foot on the ground, so the weight 



