704 A MANUAL OF PHYSIOLOGY 



cerebellar tract, the tract of Gowers, and the crossed pyramidal tract, 

 without affecting the posterior columns, tactile sensibility was only 

 slightly impaired in the opposite leg, while the sensibility for pain 

 and temperature was much enfeebled. In the left leg, which was 

 paralyzed, there was slight hyperaesthesia. These observations indi- 

 cate that impressions of pain and temperature pass up in the antero- 

 lateral column, either in the tract of Gowers, or in the direct cerebellar 

 tract, or in both (Dejerine and Thomas). But it does not follow that 

 they cannot ascend by other paths as well. It appears indeed that the 

 grey matter of the cord is such a path, and that impulses which give 

 rise to pain can be propagated along a cord in which hardly a vestige of 

 white substance remains uncut. The recent observations of Langen- 

 dorff, who found that when the blood-supply of the lower portion of the 

 spinal cord is temporarily cut off by ligation of the abdominal aorta 

 (Stenson's experiment) tactile stimulation of the posterior portion 

 of the animal causes no rise of blood-pressure and no movements, 

 have led him to the conclusion that the impulses caused by such 

 excitation have also to pass through the grey matter of the cord on 

 their way to the brain. But this conclusion cannot be accepted ; 

 for it is based on the assumption that only the grey matter, and not 

 the white, is rendered non-conducting by anaemia. Hering and 

 others have shown that the white suffers as well as the grey, although 

 it may not be so easily destroyed, and may recover more readily 

 when the blood-flow is restored (p. 659). 



The posterior columns have nothing to do with the conduction of 

 painful impressions, for division of them causes not anaesthesia, but 

 rather hyperaesthesia, while if they are left intact, and the rest of 

 the cord, including the grey matter, divided, the animal is insensitive 

 to pain below the level of the lesion. 



The impulses which descend the cord give token of their arrival 

 at the periphery by causing either contraction of voluntary muscles, 

 or contraction of the smooth muscular fibres of arteries, or secretion 

 in glands. They all pass down in the antero-lateral column, but the 

 path of the voluntary impulses in the pyramidal tracts is the best 

 known and most sharply defined. 



2. Modification of Impulses set up elsewhere (Reflex Action). 

 The spinal cord, although it is a conductor of nervous 

 impulses originating elsewhere, is by no means a mere con- 

 ductor. Many of the impulses which fall into the cord are 

 interrupted in its grey matter. Some of the efferent impulses 

 proceeding from the brain are perhaps modified in the cord, 

 and then transmitted to the muscles. Some of the afferent 

 impulses are modified, and then transmitted to the brain ; 

 some are modified, and deflected altogether into an efferent 

 path. These last are the impulses which give rise to reflex 

 effects. Strictly speaking, a reflex action is an action carried 



