THE ANATOMY OF THE HORSE 
37 
is called the gamb of the leg, and are partly attached at the 
point of the hock, the other part passing down to the fetlock 
joint through a loop, or sheath, along the back part of the 
pastern bones, and are attached to the os pedis, or foot bone. 
This muscle, from the hock down, forms one of the back ^ 
tendons of the hind leg. Another important muscle is found 
passing down underneath those already mentioned, through 
a loop, or sheath, at the back part of the hock, where it is 
supplied with a synovia sack. This is a point of importance, 
because when it becomes strained it is the seat of what is 
called thoroughpin. It then passes down the back part of 
the shin bone beneath the other tendon already mentioned, 
through the loop at the fetlock to the back part of the os 
pedis or foot bone, where it is attached. The action of these 
muscles are to flex or bend the fetlock and raise the hock 
joint in traveling. 
8. Involuntary Muscles, or muscles not under the control 
of the will. The first to notice are those of breathing or 
respiration. They form a group situated around the chest 
in such a way as to be the means of increasing or decreasing 
the size of the chest cavity. When these muscles expand 
the chest cavity is enlarged, causing the air to rush into the 
lungs, known as inspiration. On the other hand, when these 
muscles contract the air is expelled from the lungs, known 
as expiration. 
The diaphragm is a muscular curtain which separates 
the chest from the abdominal cavity. It also assists greatly 
in drawing the air in, when it contracts. This muscle also 
assists in passing manure, and in the mare foaling. It sepa- 
rates the heart and lungs from the bowels, liver and stomach. 
This muscle should be carefully examined. It can be seen 
by opening any dead animal. 
There is one muscle which is both voluntary and invol- 
untary. It is situated in the penis, surrounding the urethra, 
or the tube, which carries the urine from the bladder to the 
penis in the male animal. Its action is voluntary while the 
animal is passing urine, or water. It acts involuntary during 
sexual intercourse, forcing the semen down through the 
penis. 
