THE MUSCULAR SYSTEM 391 



machine where the strain on them is so considerable that ten- 

 dinous material is intimately mixed up with the muscular tissue ; 

 this is well seen in the masseters, the muscles of the back, fore- 

 arm, and thigh. During progression the muscular strain is 

 enormous ; for example, in the canter and gallop a weight 

 equivalent to that of the whole body is propelled by one fore- 

 leg (see chapter on Locomotion). In the horse provision is also 

 made for the muscles of the limbs being rested without necessi- 

 tating the animal assuming a recumbent position ; the mechanism 

 will be explained when dealing with the locomotion system, but 

 by means of it an animal can sleep standing, and may remain 

 standing for some weeks without serious suffering. 



Muscle Antagonism. — Every muscle, or group of muscles, 

 possesses an antagonist, and though the antagonist may be equal 

 in size, this is not always the case, as, for example, the great 

 difference between the bulk of the muscles which close the jaw 

 as compared with the trifling size of those which open it. The 

 grouping of voluntary muscles in locomotion, so as to ensure co- 

 ordination, is a question to be considered later, in the chapters 

 dealing with the Nervous System, the Senses, and Locomotion. 

 The interest which is here attached to antagonistic muscles is 

 connected with the fact that it is this antagonism which keeps 

 the muscles of the body slightly on the stretch, so that if one 

 be cut across it gapes in consequence. This elastic tension ensures 

 that no time is lost in a muscle coming into action, as there is 

 no slack to take up ; the muscle stands as it were at full cock. 



Involuntary, or Pale Muscle, is not found in masses as is the 

 red, but in thin sheets, which in places, such as the bloodvessels, 

 are only of microscopic thickness. Pale muscle is employed 

 throughout the whole length of the digestive canal from the 

 stomach to the rectum ; it is also found in the bladder, uterus, 

 spleen, and bloodvessels. In none of these places is the sharp, 

 short, active contraction of skeletal muscle required ; slow, steady, 

 deliberate movements are essential in the digestive canal ; slow, 

 steady, expulsive movements are necessary in the bladder and 

 uterus, and even in the bloodvessels, where, as we have seen, the 

 muscular tissue acts the part of a tap, it is sufficient if this tap 

 is turned on or turned off slowly and steadily. When we come 

 to study muscular contraction, it will be seen how rapidly the wave 

 passes along a voluntary, and how slowly along an involuntary, 

 muscle. 



In structure pale muscle consists of nucleated spindle-shaped 

 cells, dovetailed, and held together by a cement substance ; it 

 is through the medium of this cement substance that the wave 

 of excitation passes from cell to cell, thus forming a great contrast 

 to red muscle, where, as we shall see, the whole contracts not 



