218 VETERINARY PHYSIOLOGY 



the poison acts upon the whole length of the nerve except 

 the nerve endincfs in the muscle, stimulation of the nerve 

 still causes muscular contraction. Only when curare is 

 allowed to act upon the nerve endings in the muscle does 

 stimulation of the nerve fail to produce any reaction in the 

 muscle, while direct stimulation of the muscle causes it to 

 contract. This clearly shows that it is the nerve endings 

 which are poisoned by curare, and that therefore the appli- 

 cation of stimuli to the muscle must act directly upon the 

 muscular fibres (fig. 107), {Practical Physiology). 



(2) The action of various chemical substances also demon- 

 strates that muscle maybe directly stimulated. Thus, ammonia 

 vapour fails to stimulate nerve, but stimulates even those parts 

 of muscle which are devoid of nerve fibres. Glycerol, on the 

 other hand, stimulates nerve but does not act on muscle. 



Muscle, although it can be directly stimulated, is 

 more readily made to contract through its nerves, and a 

 knowledge of the points of entrance of the nerves into 

 muscles, the motor points, is of importance in medicine, in 

 indicating the best points at which to apply electrical stimu- 

 lation. Charts of the various parts of the body are given in 

 clinical text-books. 



2. Methods of Stimulating. 



In investigating the direct stimulation of muscle apart 

 from nerve, it is necessary to throw the nerves out of action by 

 curare, since it is most easily stimulated through nerv^e, and 

 since nerve responds to most of the stimuli which act on muscle. 



\st. Various chemical substances when applied to a muscle 

 make it contract before killing it, while others kill it at once 

 {Practiced Physiology). 



Alterations in the proportions of the salts or of the 

 cat-ions normally present in muscle — sodium, potassium, 

 and calcium — may modify its excitabilit}' and may actually 

 cause contraction. An excess of sodium ions has a special 

 action in this direction which is checked by the addition of 

 calcium ions. Potassium, on the other hand, although the 

 chief base of muscle, when added in the free ionic condition. 



