NERVE 513 



or chemical, and for both efferent and afferent nerves." The condition 

 is explainable at present only tentatively; the most likely theory, perhaps, 

 is that of electrolytic changes by the ions of the nerve-protoplasm, about 

 which little is as yet definitely known. 



Expt. 81. Afferent Impulses may Inhibit Reflexions. (Apparatus: 

 Stand-rod with femur-clamp, inductorium, key, 0.5 per cent, sulphuric 

 acid, acetic acid, beakers, small rubber rings.) (A and B of Expt. 78, 

 as well as many other experiments, indirectly indicate the nature and 

 neural mechanism of reflexions.) 



(A) Observe that a normal frog placed on its back immediately re- 

 turns, thus restoring its normal equilibrium. Now put a rubber ring 

 about the proximal part of one fore-leg. The animal no longer rights 

 its position, nor does the unconstricted leg show any tendency toward 

 moving for that purpose. This shows that the effect is in the nervous 

 system and not in the muscles. This inhibitory influence lasts only 

 about fifteen minutes, the reflex mechanism for maintaining equilibrium 

 then becoming tolerant of the unusual stimulus. In a decerebrated 

 frog the returning-reflex is too much deranged to be effective. 



(B) Pith the brain of the frog and hang up the animal by the lower 

 jaw in the clamp. Lower one foot into the beaker containing a little 

 sulphuric acid and with a watch carefully measure the time (reflex 

 reaction-time) until the leg is withdrawn. Wash off the acid carefully 

 with water and repeat the experiment. Wash off the acid again, and so 

 on ten times. Average the reaction-times. Again immerse a foot and 

 stimulate the other with a strong alternating induction-current, holding 

 the foot down for the purpose. The reflex reaction-time of the leg 

 irritated by the acid will be greatly lengthened or perhaps so strongly 

 influenced that the leg is not withdrawn at all, inhibition being com- 

 plete. , 



Expt. 82. Vaso-motor Function of Cord. (Apparatus: Stand-rod, 

 femur-clamp, and seeker.) Pith the brain only of a frog. Make a 

 small incision in the abdomen and draw out a loop of intestine; expose 

 the heart. Observe these two organs with respect to the amount of blood 

 in each, noting especially the hardness of the heart during systole and 

 the size, of the blood-vessels of the gut and omen turn. Now carefully 

 with a blunt wire seeker pith the cord of the frog. Note that the blood 

 now collects in the easily distensible omentum. 



The cord's vaso-motor centers are normally subordinate to centers 

 in the medulla oblongata, for when the local centers of the cord and 

 sympathetic ganglia are cut off from the bulb (medulla) many days 

 are necessary before the local centers in the cord by themselves take up 

 complete control. The neuraxones of probably all the spinal vaso-motor 

 cells end in sympathetic ganglia (Langley). Sometimes these cells give 1 

 out rhythmic impulses to the arterioles under their control. The vaso- 

 motor centers can be stimulated reflexly from the blood-vessels, from the 

 afferent nerves generally, or from the emotional centers probably in the 

 optic thalamus. There is an inverse relation between the surface vaso- 

 33 



