776 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



their cohesion, due to their connective-tissue constituents, is consider- 

 able. It has been found that a weight of one hundred and ten to one 

 hundred and twenty pounds was required to rupture the sciatic nerve of 

 a man at the popliteal space. The nerves lengthen very considerably 

 before breaking ; they, therefore, are extensible. 



II. NERVOUS IRRITABILITY. 



As in the case of muscle, nerves are capable of having their func- 

 tional activity called into play by various stimuli ; they are, therefore, 

 said, like muscles, to possess the power of irritability. 



Stimuli which call nervous activity into existence, like muscular 

 stimuli, may be either mechanical, thermal, chemical, electrical, or physi- 

 ological, by which is meant the normal stimuli which excite the nervous 

 system in living bodies. 



In the case of the nervous system the influence of various stimuli 

 may be made evident, either by allowing them to act upon motor 

 nerves, when the contraction of the muscles evoked will indicate the 

 stimulation of the nerve ; or, in the case of sensory nerves, by the pain 

 produced on their application. 



A mechanical irritant produces stimulation of the nerves by pro- 

 ducing change in the molecular arrangement of the nerve-particles. If 

 the mechanical stimulus, which may be of the nature of a blow, pressure, 

 pinching, and stretching, be sufficiently severe the nerve may then become 

 completely and permanently destroyed, and then lose its power at that 

 point of conducting impressions. A single mechanical stimulation of a 

 motor nerve will produce a single contraction of a muscle. If the stimuli 

 be repeated rapidty at short intervals the contractions may, as in the 

 case of electrical stimulation, be blended together, and when the stimuli 

 succeed each other more frequently than sixteen in the second a prolonged 

 tetanic contraction is produced. 



The action of variations in temperature on nerve-trunks is somewhat 

 similar to that exerted on muscles. 



If the nerve of a frog be heated to 45° C. its excitability is first 

 increased and then diminished, and the higher the temperature the 

 greater the excitability and the shorter its duration. If the temperature 

 be raised above 60° the medullary substance becomes disorganized and 

 the nerve loses its excitability. Sudden application of cold or heat acts 

 as a stimulus, and may cause muscular contraction. Increase of tempera- 

 ture above 45° produces tetanus with rapid exhaustion of the nerve. 



Anything which will rapidly change the chemical composition of a 

 nerve-trunk may act as a nerve stimulus, and, although such stimuli may 

 at first increase a nerve's excitabilit}', the}' rapidly diminish its irrita- 

 bility and often result in complete nervous paralysis. 



