THE PHYSIOLOGY OF THE BRAIN. 611 



as the anterior pyramids : most of the fibres of these are con- 

 tinuations of the pyramidal tracts of the cord and here cross 

 the middle line, forming thus the decussation of the pyramids. 

 The fibres of the direct pyramidal tract pass on in the pyramid 

 of the same side, only crossing in the cord. The pyramidal 

 fibres pass on through the pons Yarolii and along the ventral 

 or basal side of the crura cerebri (Fig. 176), and enter the 

 cerebral hemispheres. In the medulla are a number of masses 

 of gray matter (often named nuclei) which have the same 

 relation to the motor fibres of cranial nerves as areas of gray 

 matter in the cord have to the motor fibres of the spinal roots, 

 and from these motor nuclei medullated fibres join the pyr- 

 amids and go with them into the forebrain. Such fibres of 

 ascending degeneration in the cerebellar tract of the cord and 

 of the ascending antero-lateral tract as extend above the cord 

 run on the 'dorsal side of the medulla oblongata as the restiform 

 ladies ; they diverge in front so as to lie on the sides of the 

 fourth ventricle and enter the cerebellum. The fibres of the 

 posterior median column terminate in a mass of gray matter 

 in the medulla known as the nucleus gracilis : those of the 

 exterior median column in a similar nucleus cuneatus. These 

 nuclei in turn give origin to many fibres, a large number 

 crossing the middle line, and some of these are then continued 

 as the fillet along the dorsal side of the crus cerebri to the fore 

 brain; others join the restiform body and through it the 

 opposite side of the cerebellum: these crossings constitute 

 the sensory decussation, as distinguished from the pyramidal 

 or motor. The fibres of the antero-lateral descending tract 

 which do not undergo descending degeneration probably join 

 the pyramids ; all their fibres entering the medulla from the 

 cord end in gray matter of the medulla. By the word " end- 

 ing " is meant, of course, only that they cannot be further 

 traced as individual fibres, not that no physiological represen- 

 tatives of them arise in the gray matter of the medulla and 

 pass to other parts of the brain. 



The central canal of the spinal cord passes (Chap. XII) 

 into the medulla oblongata, in the anterior portion of which it 

 expands to form the fourth ventricle. The gray matter of the 

 cord is continued around the canal and on the floor and sides of 

 the ventricle ; and in connection with it are special thickenings, 

 rich in nerve-cells forming the nuclei or deep origins of most 

 of the cranial nerves: some of these nerves arise from more 



