832 THE CENTRAL NERVOUS SYSTEM 



Among the reflex activities which become excited by these nociceptive 

 impulses are those causing a rise in blood pressure — pressor impulses. 

 This correlation between nociceptive impulses and those affecting the 

 vascular reflexes has prompted 'Hanson and von Hess" 1 to make a care- 

 ful study in cats of the vascular reflexes that could be elicited from 

 various lesions in the spinal cord. Two kinds of vascular reflexes were 

 studied, pressor and depressor, the former being elicited by strong 

 and the latter by very feeble stimulation of the central end of the 

 sciatic and brachial nerves. They found that the pathways for pres- 

 sor and depressor afferent impulses were quite different. Thus, after 

 lateral hemisection of the cord, the depressor reflex obtained by weak 

 stimulation of the sciatic on the same side as the lesion was normal, 

 whereas it was greatly reduced when the sciatic nerve on the opposite 

 side from the lesion was stimulated. On the other hand, the pressor 

 reactions that were most markedly diminished were those from the 

 sciatic on the same side as the lesion. The depressor fibers evidently 

 cross in the cord, whereas the pressor do so only to a limited degree. 

 Further it was found, after cutting across the posterior part of the 

 cord, that the pressor reflexes were interfered with but not the de- 

 pressor, thus indicating that the former are transmitted either by the 

 posterior columns of white matter or by the gray matter of the posterior 

 horns. To determine which, experiments were also performed in which 

 the posterior columns were alone destroyed and the results compared 

 with others in which the tip of the posterior horn was included. Since 

 it, was only in the latter experiment that any interference with pressor re- 

 flexes was found to occur, it was concluded that the posterior horn alone 

 is concerned in the transmission of pressor impulses. 



Regarding conduction of the afferent impulses which in consciousness 

 produce pain and of those concerned in the reflex changes in respiration, 

 il was found that the posterior horn of gray matter is not concerned, 

 from which it is inferred that such impulses are conducted by the same 

 afferent path that is involved in the depressor reflex; that is to say, as 

 we have indicated above, the impulses cross in the cord to the opposite 

 side and ascend in the lateral funiculus. The pathway of the epicritic 

 and pressor sensations in the cord is not well known. It is believed, 

 however, that impulses of touch pass up the posterior column on the 

 s;ime side of the cord for four or five segments, and then gradually pass 

 to the anterior column of the opposite side. 



But for obvious reasons it is mainly from clinical observations and 

 accurate postmortem location of the spinal damage that the problem 

 must finally be solved. By these methods it has been shown that sen- 

 sations of pain and temperature pass through I he opposite lateral col- 



