THE NERVOUS SYSTEM 485 



A. Peripheral {segmental) neurones, (i) Efferent (''motor" nuclei 

 and root fibres). 



(2) Central continuations of afferent neurones (afferent roots). 

 {a) Those entering at and therefore belonging to the segment involved, 

 {h) Those entering above or below the segment and represented in 

 the segment by descending or ascending (overlapping) tracts. 



B. Terminal nuclei of (2) and the secondary tracts originating from 

 them. These may fall under category C or D (below). 



C. I ntrasegmental and Intersegmental nuclei and tracts of the seg- 

 mental brain, consisting principally of the gray reticular formation 

 and long descending tracts (arrangement much modified in forebrain). 



D. Nuclei and tracts forming parts of afferent and efferent supra- 

 segmental paths. The aft"erent paths include some of the nuclei and 

 tracts under B, and their continuations, and the efferent include some 

 of the longer systems under C, together with efferent suprasegmental 

 tracts to them. 



E. Suprasegmental structures (not present in many transections). 

 The general histology of the brain is similar to that of the cord. 



Cells of the motor cranial nuclei have an arrangement of chromophihc 

 substance similar to their analogs in the cord, while cells of aft'erent 

 cranial gangHa present a chromophihc picture similar to their cord 

 analogs. Certain cells whose axones act directly upon motor cells 

 in cord and brain {e.g., cells in motor cortex and certain cells in 

 reticular formation) resemble motor cells, in arrangement of chromo- 

 phihc substance, while certain cells in close connection with afferent 

 peripheral neurones resemble the latter. The neurogha cells and 

 fibres also present the same general characteristics as those in the 

 cord, with variations peculiar to certain localities {e.g., parts of the 

 cerebellum). 



Hindbrain or Rhombencephalon 



This includes the medulla, cerebellum and part of the tegmentum 

 and pons. Its peripheral nerves are the V, VI, VII, VIII, IX, X, 

 and XII. 1 



The jMedulla Oblongata or Bulb is the continuation upward 

 of the spinal cord and extends from the lower Hmit of the pyramidal 

 decussation below to the lower margin of the pons above. ^ 



^ It is better probably to reckon the so-called meduUarv or bulbar part of the XI with 

 the X. 



- It would be better to include in the term medulla oblongata what here falls under 

 pontile tegmentum of the hindbrain. 



