114 PHYSIOLOGY OF FARM ANIMALS [CH. 



tissue present. The following represents the approximate average 

 composition after removal of the more obvious fat : 



Protein LS per cent. 



Fat and gelatin 4 ,, 



Glycogen, ete. -9 „ 



Salts 2-1 



Water 75 



The muscles of motion and locomotion act in virtue of their 

 property of contraction. They are attached to bones by tendons, 

 one tendon being attached to one bone and the other tendon to 

 a separate bone, the muscular fibres in bet-vveen passing over one 

 or more joints. The attachment to the more stationary bone is 

 usually referred to as the origin of the muscle, while the attach- 

 ment to the more movable bone is generally called the insertion. 

 Muscles can contract to about two-thirds of their normal length. 



We may now consider very briefly some of the chief muscles 

 in the fore and hind limbs of the horse, and then pass on to the 

 mechanical laws under which these muscles work. 



The Muscles of the Fore limb. The shoulder blade is con- 

 nected to the trunk by a very strong muscle (the serratus magnus) 

 which is attached to the last five cervical vertebrae, to the first 

 eight ribs and, in the middle, to the inside of the shoulder blade. 

 When the front portion of the muscle contracts, the shoulder is 

 drawn forwards. When the rear contracts it is drawn backwards. 



The upper part of the shoulder blade is connected to the 

 trunk by a muscle attached to the inner extremity of the former, 

 and having one branch (the dorsal trapezius) going to the withers 

 and the other (the cervical trapezius) going to the suspensory 

 ligament of the neck. When the latter branch contracts it draws 

 the shoulder blade forwards. When the dorsal trapezius con- 

 tracts it has the reverse effect. 



The fore limb is drawn forwards chiefly however by a muscle 

 attached at one end to the head and top of the neck and at the other 

 end to the middle of the humerus. The fore limb is drawn back- 

 wards by the action of two muscles. The first of these is attached 

 to the sternum at one end, and to the humerus and shoulder blade 

 at the other. It pulls the limb backwards and downwards. The 

 second muscle is attached to the dorsal and lumbar vertebrae and 

 to the humerus. It pulls the limb backwards and upwards. 



When the fore limb is advanced, the shoulder blade is extended 



