The flexor muscles of the hind legs are attached, above, around the back part of the stifle joint. In tracing them down it will be found that they become tendonous. Two of the principal ones pass down to that part of the hock joint, which forms the point known as the cap. These form what is called the gambe of the leg, and are partly attached at the point of the hock, and 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 coffin bone of the foot. 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 sac. This is a point of importance, because when it becomes sprained it is the seat of what is called thoroughpin. It then passes down the back part of the shin bone beneath the other tendons already mentioned, through the loop at the fetlock to the back part of the os pedis or coffin bone, where it is attached. The action of these muscles are to flex or bend the fetlock and raise the hock joint in traveling. INVOLUNTARY MUSCLES.--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 about 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 an 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 faeces, and in the mare foaling. It separates the heart and lungs from the bowels, liver and stomach. This muscle should be carefully examined by the students. It can be seen by opening any dead animal. There is one muscle which is both voluntary and involuntary. 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. It is involuntary during sexual intercourse, forcing the semen down through the penis. [Illustration: PLATE IV. DIGESTIVE AND MALE URINARY APPARATUS OF A HORSE--AFTER MEGNIN.] EXPLANATION OF PLATE IV DIGESTIVE APPARATUS OF THE HORSE