THE NERVOUS SYSTEM 471 



the lumbar region the latter tract is no longer present and the crossed 

 pyramidal tract comes to the surface, (b) The minority of the fibres 

 of the anterior pyramids, instead of decussating, remain on the 

 same side to pass down the cord along the anterior median fissure 

 as the direct or anterior pyramidal tract, occupying a small oval area 

 adjacent to the anterior sulcus (Fig. 326). It does not usually 

 extend below the middle or lower dorsal region of the cord. As the 

 pyramidal tracts descend they decrease in size from loss of fibres 

 which continually leave them to terminate in the ventral horns. 

 The fibres of the crossed tract terminate mainly in the horn of the 

 same side, while most of the fibres of the direct tract probably cross 

 through the anterior commissure to the opposite side of the cord. 

 These tracts are thus mainly crossed tracts, as the great majority 

 of their fibres cross to the opposite side of the cord. There are, 

 however, some homolateral (uncrossed) fibres in the lateral pyram- 

 idal tract. The tracts are apt to differ in size on the two sides of the 

 cord, owing to the fact that the proportion of fibres which decus- 

 sate is not constant. The axones terminate in arborizations around 

 the motor cells of the ventral horns. The pyramidal tracts or pallio- 

 spinal system together with the spinal efferent peripheral neurone 

 system constitutes the pallio-spino-peripJieral efferent conduction path. 

 According to some authorities the pyramidal fibres terminate around 

 cells in the intermediate gray matter whose axones, which form a 

 part of the ground bundles, in turn terminate around the efferent 

 root cells. Short axone (Golgi type II) cells might also of course be 

 intercalated in this connection. (Figs. 330 and 331.) 



The pyramidal tracts convey to the cord the impulses which result in volun- 

 tary movements, especially, probably, individual movements of parts of the 

 limbs (foot, hand, finger, etc.). 



n. The Colliculo -spinal Tract {Tectospinal Tract) orginates in 

 the colliculi of the midbrain roof, decussates and descends to the cord, 

 where it lies near the ventral sulcus. Its presence in the cord has 

 been disputed. (Fig. 358.) 



in. The Tract from the Nucleus of the Posterior or Medial 

 Longitudinal Fasciculus. — This nucleus is located in the reticular 

 formation of the tegmentum of the midbrain cephalad to the nucleus 

 of nerve III.^ The tract originating from it forms in the brain a part 



^ The fibres in question have been variously stated to originate from the nucleus of 

 Darkschewitsch, the "nucleus of the posterior longitudinal fasciculus," and the nucleus 

 of the posterior commissure. Whether any of these nuclei is the same as the interstitial 

 nucleus of Cajal, or the nucleus of van Gehuchten in fishes, is uncertain. By the nucleus 

 of the medial longitudinal fasciculus is here meant the interstitial nucleus of Cajal. 



