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.
DIGESTIVE AND MALE URINARY APPARATUS OF A HORSE—AFTER MEGNIN.
EXPLANATION OF PLATE IV
Digestive Apparatus of the Horse