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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



nearly destroyed, while the voluntary movements of the leg were but slightly 

 disturbed (Fig. 177). After section of the entire cord, with the exception of 

 the dorsal columns, both sensation and motion were lost (Fig. 178). 



FIG. 176. Outline of the 

 spinal cord of a dog; 

 the shaded portion indi- 

 cates the extent of the 

 lesion. The lateral col- 

 umns of the cord are in- 

 tact (Osawa). 



FIG. 177. Outline of the spi- 

 nal cord of a dog ; the shaded 

 portion indicates the extent 

 of the lesion. The dorsal and 

 ventral columns, together 

 with the intermediate gray 

 matter, are intact (Osawa). 



FIG. 178. Outline of the spi- 

 nal cord of a dog ; the shaded 

 portion indicates the extent 

 of the lesion. The dorsal 

 columns alone are intact 

 (Osawa). 



Here are a number of very striking results. It is to be noted that the 

 lateral columns of the cord form the important pathway for all the impulses 

 which influence sensation and motion caudad to the section, but, at the same 

 time, section of them causes a marked diminution of sensation alone. On the 

 other hand, the preservation of the dorsal columns alone does not preserve 

 sensation. 



It will be understood, of course, that the motion in question is executed 

 by muscles lying caudad to the section and is co-ordinated with that of the 

 structures lying in front of it. Similarly sensation was inferred from move- 

 ments executed in front of the level of the section and caused by stimulation 

 behind it. 



A double hemisection of the spinal cord as described above seems to involve 

 an interruption of all the long pathways. Yet the nervous impulses pass 

 such a block in both directions. Probably within the central system as 

 elsewhere the amount of information conveyed is not directly dependent on 

 the number of nerve- fibres stimulated. In general, a very small number 

 those brought into action by pulling out a single hair are as efficient in 

 co-ordinating our responses as would be the stimulation of a thousand times 

 the number. Such being the case, it is not impossible that although after the 

 sections of the cord both the number and intensity of the impulses that pass 

 the point of section may be diminished, yet they may still remain sufficient to 

 modify the reactions of the caudal portion of the cord, which is in no very 

 great degree dependent on such modifying impulses. That the impulses may 

 pass along a cord twice hemisected on opposite sides demands the aid of the 

 gray matter, and we at once refer to the short fibre-tracts as the pathway. 



It is a drawback to such a view that physiologists have not been accustomed 

 to lay much weight on the connections established by these short tracts, but 

 from the anatomical side there is no inherent difficulty in accounting for many 



