S54 TEXT-BOOK OF PHYSIOLOGY 



3. The Tecto-spinal Tract, or Tract of Lowenthal. The fibers of this tract 



have their origin in cells of the corpora quadrigemina, from the tectum 

 or root of the midbrain. After their emergence they cross the median 

 plane, descend the medulla and come to occupy a position in the ven- 

 tral funiculus. Though its exact location is a matter of doubt, it is 

 probably in close relation to the ascending spino-tectal fasciculus. The 

 fibers terminate around certain cornual cells. 



4. The Olivo-spinal Tract, or Bundle of Helwig. This tract was alluded 



to on page 552 as composed of ascending fibers. It, however, con- 

 sists in part of descending fibers which arise in the cells of the inferior 

 olive. They serve to associate the cells of the olive with the cornual 

 cells in the cervical and upper thoracic regions. 



5. The Vestibulo-spinal Tract, or the Ventro-marginal Tract. The fibers 



of this tract have their origin in the cells of the primary centers of the 

 vestibular nerve. As they descend they come to occupy a region 

 on the surface of the ventral funiculus. The terminals of these fibers 

 become related to cornual cells at various levels of the cord. 

 The fibers composing the last three tracts are more or less inter- 

 mingled and hence their topographical distribution cannot be stated 

 other than approximately. 



THE FUNCTIONS OF THE WHITE MATTER 



The function of the white matter in general is the conduction of nerve 

 impulses both up and down the cord. By reason of the origin, course and 

 termination of the fibers composing it this general function of the white 

 matter may be regarded from at least three points of view, viz., 



1. Intersegmental conduction mediated by the fibers composing the 



.fasciculi proprii. 



2. Ascending or spino-encephalic conduction mediated by the spino- 



encephalic tracts or fasciculi. 



3. Descending or encephalo-spinal conduction mediated by the encephalo- 



spinal tracts or fasciculi. 



Intersegmental or Associative Conduction. The spinal cord con- 

 sists of a series of physiologic segments each of which has specific functions 

 and is associated through its related spinal nerve with a definite segment 

 of the body. For the harmonious cooperation and coordination of all the 

 body segments it is essential that the spinal segments should be united by 

 commissural or associative fibers. This is, in fact, accomplished by the 

 axons of the intrinsic cells of the gray matter, which constitute such a large 

 part of the ventro-lateral and dorsal fasciculi proprii. In consequence of 

 this association, the cord becomes capable of complex coordinated and pur- 

 posive reflex actions. 



Ascending or Spino-encephalic Conduction.- The conduction of nerve 

 impulses from below upward necessitates the existence of tracts or fasci- 

 culi of nerve-fibers which extend from different levels of the spinal cord 

 to associated but more or less widely separated regions of the encephalon. 

 The origin, course and termination of these ascending tracts as established 

 by different methods of investigation have been stated on page 551. The 

 functions of these different tracts are best understood when their relations 

 to the dorsal roots of the spinal nerves are kept in view. It will be re- 



