386 THE NERVOUS SYSTEM. 



cord while passing from the cervical region to the medulla oblongata. 

 At the cervical enlargement in the lower part of the neck (Fig. 102, II.) 

 the cord is very wide transversely, owing partly, no doubt, to the root 

 fibres of the great nerves of the brachial plexus, which have joined the 

 lateral columns after traversing the gray substance of the anterior 

 horn. But at the upper extremity of the cord (Fig. 102, I.) its trans- 

 verse diameter diminishes and its antero-posterior diameter increases ; 

 since some of its fibres have left the lateral column to reach an anterior 

 position on the opposite side. 



Beside the decussating fibres of the pyramids derived from the lateral 

 columns of the cord there are others which come from the posterior 

 columns and the posterior horns of gray substance. The change in 

 direction of these fibres takes place at a little higher level than that 

 just described. It forms the upper portion of the decussation of the 

 pyramids. The fibres, after leaving the posterior columns and horns, 

 run forward and inward, cross the median line obliquely, like the pre- 

 ceding, and then join the anterior pyramids, forming their deeper and 

 more lateral portions. As the pyramids reach the tuber annulare, 

 therefore, they are composed superficially and toward the median line 

 of fibres from the opposite lateral columns of the cord ; while their 

 deep-seated and external fibres come from the opposite posterior col- 

 umns and horns. 



The further continuation of the anterior pyramids is through the 

 tuber annulare into the crura cerebri, of which they form the lower- 

 most or superficial portion. This part of the crus cerebri, which is that 

 visible at the base of the brain, sends its fibres mainly forward, upward, 

 and outward, into the substance of the corpus striatum. But accord- 

 ing to Huguenin a portion of the fibres on its external border, repre- 

 senting those which have come from the posterior columns and horns, 

 pass behind the cerebral ganglia to reach the convolutions of the oc- 

 cipital lobe. 



Deep-seated Portion of the Crura Cerebri. The deep-seated or 

 uppermost portion of the crura cerebri, is formed of fibres from the 

 anterior columns of the cord, and from the anterior part of the lateral 

 columns. The anterior columns of the cord are contiguous to the 

 median fissure until their place is taken, as above described, by the 

 obliquely decussating bundles of the anterior pyramids. In the me- 

 dulla oblongata, they thus come to be placed farther outward and 

 backward, and in passing through the tuber annulare they occupy a 

 deep-seated position in its interior. Thence they run forward in the 

 upper or deep-seated portion of the crura cerebri, and pass to the optic 

 thalami. The remaining fibres of the lateral column, which have not 

 taken part in the formation of the pyramids, continue their course 

 upward, pass through the medulla oblongata and tuber annulare, and, 

 finally, joining the deep-seated portion of the crura cerebri, reach in 

 this way the optic thalami. 



Inferior Peduncles of the Cerebellum. The inferior peduncles of 



