I 



82 THE NERVOUS SYSTEM 



Starr, so called because up this column pass the impulses coming from 

 the basal or vegetative organs of the body, by which impulses, reflexly, 

 the centers above control their actions. Posteriorly lies the posterior 

 external column (Burdach's) on the inner side of the posterior horns. 

 Some of these fibers end about the cuneate nucleus of the medulla. 

 They conduct tactile and muscular impulses afTerently from the pos- 

 terior roots connected with the arms and neck. Others are association- 

 fibers between various spinal segments, while others still, more or less 

 horizontal in direction, convey messages from posterior roots to cells in 

 the gray matter of the cord. Medial to this column and bounded within 

 by the posterior fissure is the posterior-median column (Goll's) which 

 more or less similarly conveys important (sensory, afferent) impulses 

 from the skin, muscles, and joints of the legs and the lower part of the 

 trunk, upward to the gray nuclei of the medulla's nucleus gracilis. The 

 paths of this tract communicate more or less with the gray horn of the 

 same side of the cord. A distinct fibrous septum, especially above, 

 separates this column from the preceding. On the medial side of this 

 tract are two smaller tracts bordering on the posterior fissure : posteriorly 

 the septo-marginal bundle, and just anterior to it, and sometimes extend- 

 ing to the posterior commissure, the so-called oval bundle. These are 

 probably association-paths, as is also the so-called comma-tract (afferent) 

 in the anterior part of the posterior external column. Close to and 

 within the place of exit of the posterior roots from the cord is the pos- 

 terior marginal tract (Lissauer's), a small bundle composed of fibers 

 rising in the posterior horns and extending both upward and downward 

 to connect with the posterior columns and with other posterior roots. 

 This, then, is also an association-bundle. It is likely that in addition 

 to these more or less well-defined tracts there are others and especially 

 many small association-paths. The latter are particularly on the cord's 

 periphery, and serve to unify to an unknown extent all the various 

 parts of the cord, as similar fibers unify the brain. The functional 

 facts demand this supposition, and many experiments, especially those 

 of Sherrington and of Flechsig, indicate strongly their existence.' 



DISTRIBUTION AND COLLECTION. These are together functions 

 general to the spinal cord, and we have seen well enough in the last 

 few paragraphs how they are brought about. The cord has as its 

 major part thick conducting columns composed of numberless fibers 

 and fibrils which often run completely through the trunk from end to 

 end. Within this layer of white columns is a mass of gray matter 

 composed in part of nerve-cells and of short but complicated neurones. 

 These, as we have seen, associate freely, by means of their dendrites, 

 neuraxones, and countless collaterals. Down the cord so constituted 

 come numberless and most various impulses, to be distributed wherever 

 normal function demands, but what determines just where, and what 

 determines where not (Morat calls this viatility), we do not as yet 

 know. In like manner countless impulses pass into the cord, to be dis- 

 tributed in a way more, or less similar. Again, numerous influences 

 enter the cord and are made to concentrate or to collect so that their 



