944 THE RETICULAR FORMATION. [BOOK in. 



cervical spinal cord and beginning bulb, are reduced to small 

 dimensions and in the end disappear ; but before we speak of the 

 course and fate of the tracts of fibres constituting these columns 

 we must turn to the important changes of the grey matter. 



608. A transverse section through the lower end of the 

 decussation (Fig. 109, 1) shews, as we have said, few differences 

 as regards the grey matter from one taken at the level of the 

 second cervical nerve. The changes noticeable are mainly the 

 changes in position of the posterior horns, the increase of central 

 grey matter around the central canal, the approach of the lateral 

 horn, from which spring the roots of the spinal accessory nerve, 

 to the anterior horn, and an increase of the reticular formation 

 in the bay ventral to the posterior horn. 



In the middle of the decussation (Fig. 109, 2) the decussating 

 fibres are cutting the head of the anterior horn away from the 

 base of the horn and the central grey substance, and the isolated 

 head is diminishing in size, being separated from the surface of 

 the cord by an increasing thickness of white matter. The lateral 

 horn and origin of the spinal accessory root do not share in this 

 isolation, but are driven back again dorsally towards the posterior 

 root to join the reticular formation which is increasing in area, 

 while the lateral column of white matter is diminishing in bulk 

 by the withdrawal of the pyramidal tract. 



Still a little further forward, the anterior horn seems at first 

 sight to have wholly disappeared (Fig. 109, 3 and 4), but its 

 disappearance is coincident with an increase of the reticular 

 formation in the position of the lateral columns, as well as with 

 the growth of tissue mentioned above between the anterior fissure 

 and the central grey matter. In fact, between the anterior pyra- 

 mids on the ventral side and the largely increased and laterally 

 expanded grey matter on the dorsal side, a large area of peculiar 

 tissue now extends on each side for a considerable distance from 

 the middle line of the raphe, encroaching on what was the lateral 

 column of white matter; and a corresponding area of similar 

 tissue may be traced from this level through the higher parts of 

 the bulb up into the pons and crura cerebri. The tissue consists 

 of nerve fibres running transversely, longitudinally, and in other 

 directions, so as to form a network, the bars of which are often 

 curved ; and with these fibres are found branched nerve cells in 

 considerable number, some of them small, both fibres and cells 

 being as elsewhere embedded in neuroglia. Though differing 

 from the ordinary grey matter of the cord by the more open 

 character of its network, it may be considered as a form of grey 

 matter. We may consider it as being in reality the grey matter 

 of the apparently lost anterior horn broken up and dispersed by 

 the passage of a large number of fibres and bundles of fibres, 

 especially of the decussating fibres spoken of in 606, which 

 since they curve through this area from the middle line laterally 



