THE MUSCULAR SYSTEM 53 



body weight (Fig. 8). Muscle tissue is responsible for all move- 

 ments or work, both voluntary, like walking and jumping, and 

 involuntary, like the circulation of the blood and movements in all 

 the other internal organs. Since they must be active both night 

 and day, muscles are classed with the nerves as the master tissues 

 of the body. 



There are two kinds of muscles: (1) voluntary or striped, and 

 (2) involuntary, unstriped or smooth. 



VOLUNTARY MUSCLES 



The voluntary muscles are under the control of the will. They 

 are often referred to as skeletal muscles, for they act on the bony 

 levers. In general, their color in the horse is dark red or even 

 brown. When exposed to the air they acquire a bluish luster. 

 There is a wide difference in the volume or size of voluntary muscles: 

 some are very strong and powerful, like those of the thigh, while 

 others are short, delicate strands, capable of doing but little work. 

 The form varies with the work to be performed by the muscle. 

 The usual forms are long, short, and ring-like. Depending on 

 shape, voluntary muscles are classified into simple and digastric; 

 biceps, triceps, and quadriceps; penniform and semipenniform. 

 The structure of voluntary muscles is seen upon microscopic 

 examination to be bundles of extremely delicate fibers which 

 are striped horizontally. Each fiber has a delicate covering 

 membrane, the sarcolemma. Bundles of fibers are held together 

 by the perimysium, which is composed of bands of white fibrous 

 tissue. When much strain is placed on a muscle, tendinous 

 fibers are mixed with the fleshy fibers. Blood-vessels and nerves 

 are plentifully distributed throughout the musculature (Fig. 9). 

 Fresh horse meat possesses a peculiar, sweet, somewhat disagree- 

 able odor. 



Tendons are round or flattened cords fitted to the extremities 

 of long muscles. Flat muscles have widened tendons, named 

 aponeuroses, for attachment. A tendon is usually less extensive 

 than the fleshy portion of the muscle; this permits many muscle 

 attachments to a relatively small surface. In the horse's legs are 

 found "check ligaments," more properly called tendons, which 

 take the strain off the muscles of the limbs and allow the animal to 

 sleep in the standing position without undue fatigue. 



The attachments of a muscle determine its extent, direction, 



