THE MEDULLA OBLOXGATA AND BEAIN STEM 487 



addition of many fibres coming down from the medulla and cere- 

 bellum, which place the nerve centers of the spinal cord in close 

 relation with those of the cranial nerves and with the association 

 centers of the cerebellum. 



In addition to the large size of its white columns, a noticeable 

 characteristic of the upper cervical region is the prominence of its 

 lateral horns of grey matter. Just dorsal to the lateral horns is 

 also a peculiar reticular formation which results from an invasion 

 of the adjacent portions of the lateral white columns by bands of 

 grey matter. The grey matter thus forms a coarse network whose 

 meshes inclose isolated bundles of longitudinal nerve fibres. 



The ventral horn cells of this region are scarcely divisible into 

 groups, but a large and distinct cell group, the inter medio-lateral 

 cell column^ occupies the so-called lateral horn. 



The nuclei of the upper cervical region enervate the skin and 

 muscles of the neck and shoulder, they also supply the diaphragm. 

 The nerve cells of this region not only supply the cervical spinal 

 nerves, but they also send root bundles to the spinal accessory or 

 eleventh cranial nerve. 



THE MEDULLA OBLONGATA AND BRAIN STEM. Passing 

 from the first cervical segment of the spinal cord to the lower 

 portion of the medulla, a remarkable rearrangement of the central 

 grey mass is noticed, the change being apparently dependent upon 

 three prime factors, as follows : 



1. The passage of numerous bundles of white fibres between 

 .the lateral columns of the spinal cord and the anterior columns 

 or pyramids of the medulla, cuts off the ventral horns of the grey 

 matter ; the detached ventro-lateral portion with its lateral cell 

 groups is thus displaced lateral ward, while the base of the ventral 

 horns with their mesial cell groups remain adherent to the grey 

 commissure near the region of the central canal. Between these 

 two portions of the ventral horns is an irregularly disposed and 

 interlacing mass of nerve fibres, interspersed with fragments of 

 grey matter, which forms the formatio reticularis of the medulla 

 oblongata. 



2. The anterior median fissure in this region becomes merely a 

 shallow and deep raphe across which many nerve fibres decussate. 

 Those long fibre bundles which in the spinal cord occupied the 

 lateral and posterior columns are now found in the anterior or 

 ventral portion (pyramids, formatio reticularis, and fillet) of the 

 medulla oblongata. 



