470 THE ORGANS 



through the intermediate gray matter, possibly group a (p. 456 and 

 Fig. 319). Some fibres come from tautomeric, others from hetero- 

 meric cells, the axones of the latter crossing in the ventral commis- 

 sure. The tract first appears in the upper lumbar cord and natur- 

 ally increases in size as it passes upward. The fibres of this tract 

 also end in the vermis of the cerebellum. They reach their desti- 

 nation in the cerebellum by a dift'erent route, ascending considerably 

 farther than the dorsal spino-cerebellar fibres and then turning back 

 along the outer side of the superior cerebellar peduncle to the ver- 

 mis. This path is thus also a two-neurone path (spinal ganglion 

 cells and spino-cerebellar neurones) and is partly crossed and partly 

 uncrossed. The ventral spino-cerebellar and spino-thalamic tracts 

 are sometimes referred to as Gower's tract. (Figs. 331 and 345.) 



It seems probable that muscle-tendon sense passes up the cord by tract I 

 (uncrossed in the cord), while pain and temperature pass up by the spino- 

 thalamic tract (crossed). The path pursued by touch is more doubtful but it 

 may pass up partly by tract I and partly by ascending arms of varying lengths 

 which end in the cord around heteromeric column cells (thus partly uncrossed and 

 partly crossed in the cord). This path may join the fillet in the medulla. It 

 would seem probable that the cerebellar tracts convey stimuli from muscle- 

 tendon receptors. Ascending paths may also be formed by successive relays of 

 shorter tracts in the ground bundles of the cord and the reticular formation of 

 the brain. This has been especially claimed for the pain pathway. The afferent 

 visceral path to the pallium is not known. 



Descending Tracts 



I. The Pyramidal Tracts {Tradus Cortico-spinalis, Cerebro- 

 spinalis or Pallio-spinalis) .—The cell bodies of the neurones whose 

 axones make up this system are situated in the cerebral cortex 

 anterior to the fissure of Rolando (precentral area, Fig. 365). Their 

 axones converge in the corona radiata and pass downward through 

 the internal capsule, pes pedunculi, pons, and medulla, sending off 

 fibres to the motor nuclei of the cranial nerves. In the medulla 

 the tracts come to the surface as the anterior pyramids. At the 

 junction of medulla and cord occurs what is known as the pyramidal 

 decussation. Here (a) most of the fibres of each tract cross to the 

 opposite dorso-lateral region of the cord and continue downward 

 as the crossed or lateral pyramidal tract. This Hes in the dorsal part 

 of the lateral column (Figs. 319 and 326). It extends to the lower- 

 most part of the cord. In the cervical and dorsal region it is sepa- 

 rated from the surface of the cord by the direct cerebellar tract. In 



