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in the same nerve trunk. Upon stimulation the effects of one 

 may be entirely masked by the effects of the other, so that 

 it becomes necessary to learn the differences between the two. 



1. The vasoconstrictors are excited less easily than the 

 vasodilators. 



2. The after-effect of stimulation of the constrictors is 

 shorter than that of the dilators. 



3. Warming increases the excitability, and cooling decreases 

 it, more in the constrictors than in the dilators. 



4. The maximum effect of stimulation is reached more 

 quickly in the constrictors than in the dilators. 



5. The constrictors have a latent period of 1.5 seconds; that 

 of the dilators is 3.5 seconds. 



The Vasomotor Centre. There is in the medulla on each side 

 of the median line a group of cells lying in the region of the 

 nucleus of the facial and of the superior olive. This is the 

 situation oJ the vasomotor centre. It is bilateral, and occupies 

 an area caudal to the corpora quadrigemina. When sections 

 are made through successive levels of the bulb, the pressure 

 of the blood begins to fall when a point is reached about 1 mm. 

 caudal to the quadrigemina, and continues to fall until an area 

 extending over the fourth millimeter has been reached. There 

 is then no further fall. The centre continually sends impulses 

 along fibers that extend to the nuclei of various cranial nerves, 

 and also down the lateral columns of the cord to small cells 

 situated at various levels in the anterior horn and lateral 

 gray substances. From these cells axis cylinders pass out 

 through the anterior roots of the cranial and spinal nerves 

 and enter the sympathetic ganglia. Here cells, in turn, send 

 out processes that end on the muscle fibers of the bloodvessels. 

 The evidence for the existence of subsidiary spinal centres in- 

 conclusive. It has been found that in a dog whose cord is severed 

 at the junction of the dorsal and lumbar regions mechanical 

 stimulation of the skin of the abdomen and penis will cause 

 erections. This is a vasomotor reflex due to dilatation of 

 bloodvessels of the penis through the nervi erigentes. Again, 

 section of the cord in the dorsal region of a dog is followed 

 by a vasodilatation of the arteries of the hind limbs. If the 

 animal continues to live, the limbs are in time restored to their 

 normal condition. Destruction of the lumbar cord now brings 



