CHAPTER IV. 



Medulla Oblongata — General Somatic Afferent 

 System. 



THE transition from the spinal cord to the medulla oblon- 

 gata is gradual, the various parts changing their form 

 and relations by degrees. The boundary is usually con- 

 sidered arbitrarily to be at the decussation of the pyramids. 

 The greater part of the medulla oblongata contains the wide 

 fourth ventricle and is known as the open portion, so that the 

 section differs greatly in outline from sections of the spinal 

 cord, being relatively broad and low (dorso-ventrally) with a 

 hollow in the dorsal region representing the fourth ventricle. 

 This concavity is lined with gray matter which corresponds 

 to the central gray matter surrounding the canal of the 

 spinal cord, and in which various special groups of cells appear. 

 Extending right through the section in the median plane is a 

 partition formed largely by decussating fibres, the raphe. 

 Round the periphery of the section appear a number of more 

 or less sharply marked off tracts and nuclei, such as the 

 spinal V root and its nucleus, and between these and the 

 raphe is a large area of intermingled gray and white matter 

 known as the substantia reticularis or reticular formation. In 

 a zone extending dorso-ventrally along each side of the raphe, 

 the white fibres predominate, and hence this area is called 

 the substantia reticularis alba, while the remainder of the for- 

 mation, in which there is a larger proportion of gray matter, 

 is the substantia reticularis grisea (PI. VII.). 



In the cranial nerves, three of the four primary functional 

 types are represented by both general and special subdivisions, 

 while the somatic motor group has special elements only, 



