The Muscular System. 245 



varies with the tension of the muscle : the higher the 

 tension the greater the temperature. This has been pointed 

 out as the probable explanation of the high temperatures 

 registered in cases of tetanus. Colin found that the tem- 

 perature of the masseter muscle of the horse rose 5° Fahr. 

 through feeding. The more fatigued a muscle becomes 

 the less heat it evolves. 



Muscles possess, in common with some other tissues of 

 the body, the phenomenon of Irritability or Excitability, 

 viz., the power of contracting when irritated ; the movement 

 in muscle shows itself by the contraction or shortening 

 which occurs. The normal stimulus to muscle is imparted 

 to it through the nerves, but chemical, mechanical, and 

 electrical stimuli also lead to a contraction even in the 

 absence of nerves, or when applied to a muscle removed 

 from the body. 



Through the sensory nerves the brain is made acquainted 

 with the position of the body and the state of the muscles, 

 viz., relaxed or rigid, cramped or fatigued. The amount of 

 ordinary sensibility in muscle is not very great unless it be 

 cramped or inflamed. Under these conditions acute mus- 

 cular pain may be manifest, as we see in the painful lameness 

 of embolism. The special function, however, of the sensory 

 nerves is communication between the exterior and the 

 brain, keeping the latter acquainted with the condition of 

 the former. 



By means of the motor nerves the muscles are supplied 

 with the needful stimulus which brings about contraction ; 

 division of the motor nerves or interference with their 

 function leads to muscular paralysis, of which the best 

 example in the horse is the left laryngeal paralysis result- 

 ing from pressure on the recurrent nerve. 



When a muscle contracts it becomes shorter and thicker ; 

 the exact microscopical changes which occur are disputed, 

 but no doubt the transverse striae are brought closer 

 together ; after contraction it returns to its original length. 

 Contraction after contraction may be produced, until at 

 last the muscle becomes fatigued ; it now contracts more 



