42 SKETCH OF THE ANATOMY OF THE HORSE. 



is either flexed or extended. The other joints of the hock possess hardly 

 any power of movement. 



The astragalus, os calcis, small bones of the hock, hind cannon bone, and 

 splint bones are firmly connected together by ligaments. 



The joints below the hock are similar to those below the knee. 



Muscles. — The principal muscles which are used in locomotion 

 will now be briefly considered. 



A broad sheet of muscle (the pannicidus) lies immediately underneath 

 the skin that covers the neck, sides of the chest, and abdomen. In thin 

 horses, its rear border is usually defined by an irregular line (p. 355) 

 which runs along the side downwards and backwards towards the groin. 

 This muscle is attached, round its borders, to the skin and superficial 

 muscles by sheets of fibrous tissue. By quickly contracting and re- 

 laxing alternately, it causes the skin to twitch, and thus gets rid of flies, 

 etc., that may have alighted on the surface underneath which a portion 

 of this muscle lies. It is principally found on those parts which the 

 horse has difficulty in reaching with his Hps, tail, or mane. As the 

 process of training for racing purposes appears to largely develop this 

 muscle, I cannot help thinking that it aids in forced expiration — 

 expelling the air from the lungs — during the quickened breathing 

 entailed by fast work ; although writers on anatomy do not ascribe 

 this action to it. 



The neck, back, and loins are flexed by muscles which lie immediately 

 underneath the spine. The head is bent by muscles that proceed — one 

 on each side — from the breast-bone to the lower jaw, and by others 

 which connect the neck and head together. The head is extended by 

 muscles that are attached to the poll and bones of the neck and by others 

 that proceed from the poll to the withers. The neck is extended by the 

 last mentioned muscles, and by those which connect the spine of one 

 vetebra with the body of the one in front of it. The hack and loins 

 are extended by muscles which are similar in action to the one just alluded 

 to, and by the longissimus dorsi, which is the most powerful muscle 

 of the body, and is the chief extensor of the spine. It forms the principal 

 portion of the fleshy mass which lies over the loins and back. It is 

 attached to the pelvis, sacrum, all the loin and dorsal vertebrae, the last 

 four bones of the neck, and to the ribs. As the spines of the vertebrae 

 (including the withers) form a part of its attachments, it follows that 

 the more they are developed, the more powerful will" be the extension of 

 the loins and back. 



Muscles which connect the fore limb to the body. — The shoulder-blade is 

 connected to the trunk principally by a very strong, fan-shaped muscle 

 (serratus magnus), which is attached at its middle to the inside of the 

 shoulder-blade. Its front end is connected to the last five bones of the 



