166 GENERAL PROPERTIES OF LIVING TISSUES 



lus, whether chemical, mechanical, or electrical. 

 There are reasons for believing that the rhythmi- 

 cal contractions of the heart are the consequence 

 of a constant chemical stimulus. 



MECHANICAL STIMULATION 



V 



Stimulate a nerve mechanically by pinching 

 the cut end with forceps. 



No change will be seen in the nerve, but the 

 muscle will shorten, and then relax. 



Mechanical stimulation has the advantage that 

 it can be localized accurately, and for this reason 

 it has been used where electrical stimulation 

 seemed inapplicable. Tetanomotors have been 

 constructed by Heidenhain and others to give a 

 Tapid succession of slight blows upon the nerve. 



Sudden pressure on a muscle or sudden exten- 

 sion may cause contraction. Sometimes the 

 whole muscle contracts; sometimes only the 

 portion directly stimulated. 



Idio-Muacular Contraction. With the point 

 of the seeker stroke the diaphragm and other 

 muscles of a recently killed rat, or other small 

 warm-blooded animal, in a direction at right 

 angles to the course of the fibres. 



A wheal, i. e. a long-continued shortening and 

 thickening of the fibre stimulated, will be seen. 

 If the animal be not too long dead, a momentary 



