60 PBINCIPLES OF VETERINARY SCIENCE 



in groups of flexors and extensors that oppose each other so that 

 the contraction of one group moves the bone against the pull of 

 the extensible and elastic antagonist. This enables the muscles 

 to move the bony levers with greater facility, smoothness, and 

 effectiveness. The accompanying charts show that muscle 

 elasticity is not like that of a dead elastic body (Fig. 14) . 



Muscular contraction depends upon the properties of irrita- 

 bility and contractility, both of which are well developed in the 

 muscles. Irritability is the property of response to stimulation 

 by outside obj ects. Contractility is the property of muscle which 

 is manifested after it receives a stimulus. It results in the 

 muscle shortening in length and increasing in thickness, i.e., the 

 two ends are brought closer together. It is normally due to a 

 stimulus received from the central nervous system through motor 

 nerves. When these nerves are severed, the muscle is said to 

 be paralyzed. However, experiments with thermal, mechanical, 

 and electrical stimuli show that muscles, whose connection with 

 the central nervous system is cut off, are capable of independent 

 contraction. Thermal stimulation resulting in contraction is 

 seen when an animal is butchered — the cold air coming in contact 

 with the recently exposed flesh sets up a pronounced twitching. 

 Another example is "shivering" due to sudden exposure of the 

 skin to cold. Mechanical stimuli, such as a sharp blow or a 

 pinch, will cause a contraction in the muscles. Electrical stimuli 

 are most suited for the experimental study of the duration, 

 extent, speed, and strength of contraction, because their intensity 

 can be controlled so as not to injure the muscle substance. 



The duration of a simple contraction varies in muscles from 

 different parts of the same animal. There is little difference, 

 however, in the length of the period in long and short muscles, as 

 they contract throughout their entire extent at nearly the same 

 moment Ordinarily it does not last longer than the fraction of a 

 second. Contraction may be voluntarily maintained for a 

 considerable period and is shown by the muscle being hard to the 

 feel. Relaxation always follows contraction so that the muscle 

 may recuperate. 



The extent of contraction is about one-third the length of the 

 muscle, e.g., a muscle 3 inches long can shorten 1 inch, while 

 one 6 inches in length can shorten 2 inches. In other words, 

 the range of contractility in a long muscle is greater than in 



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