VASOMOTOR REFLEXES 377 



blood-pressure due to this cause does not occur. Mechanical 

 interference with the circulation as a result of the increased 

 movements of the thoracic walls seems to be the main cause of 

 this fall, since it can be eliminated by opening the thorax. 

 When this complication is not taken into careful consid- 

 eration the results of vasomotor experiments are liable to be 

 misinterpreted. 



2. In dogs under ether, chloroform, or brain compression, a 

 weak stimulation of the central end of the cut nerves (sciatic, 

 saphenous, tibial, peroneal, median, ulnar, and vagus) pro- 

 duces usually a fall and a strong stimulation, a rise of blood- 

 pressure. With a gradual increase of the strength of stimulus 

 up from the threshold, the reflex fall of blood-pressure first 

 increases, then decreases, and gradually becomes converted 

 into a rise, passing through a neutral point. We have failed to 

 obtain a pressor effect by the strongest stimulation of the de- 

 pressor nerve of Cyon. 



3. The frequency of stimulation has an effect upon vasomotor 

 reflexes. With a rapid rate of stimulation a rise is obtained 

 and with a slow rate of stimulation in many cases a fall of blood- 

 pressure. Of the different rates of stimulation w^e employed 

 (one to eighty per second), the maximum pressor response is 

 reached at twenty to forty per second. 



4. No essential qualitative difference was found among vari- 

 ous nerves (sciatic, tibial, peroneal, median, ulnar, branch of 

 femoral nerve) subjected to stimulation. The saphenous nerve 

 has a greater tendency to give a fall than those above mentioned. 

 A purely sensory nerve seems to have a somewhat lower thresh- 

 old than other kinds of nerves. Between nerves of the same 

 category but of different sizes, the larger one produces usually 

 a more marked response within a limited range of the strength 

 of stimulation. 



5. When the animal is under ether, chloroform, or brain com- 

 pression, stimulation (mechanical, thermal, chemical, and elec- 

 trical) of nerve terminations, such as those in the skin, muscles, 

 and the intestine, causes a fall of blood-pressure in the great 

 majority of cases, but violent or extensive stimulations of the 



