THE BULB. 595 



in previous sections surrounded it, is now exposed to the surface on the 

 floor of the ventricle, the median posterior columns being thrust aside. In 

 a still more forward section (Fig. 132, 6) this gray matter in correspondence 

 with the increasing width of the ventricle occupies a still wider area, thrust- 

 ing still further aside the narrowing upper ends of the two posterior 

 columns. 



During these successive changes the large, wide posterior (both external 

 posterior and median posterior) columns of the cervical spinal cord and 

 beginning bulb, are reduced to small dimensions and in the end disappear ; 

 l)ii t before we speak of the course and fate of the tracts of fibres constitu- 

 ting these columns we must turn to the important changes of the gray 

 matter. 



521. A transverse section through the lower end of the decussation 

 (Fig. 132, 1) shows, as we have said, few differences as regards the gray 

 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 gray 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. 132, 2) the decussating fibres are 

 cutting the head of the anterior horn away from the base of the horn and 

 the central gray substance, and the isolated head is diminished 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 toward the poste- 

 rior 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. 132, 3 and 4), but its disappearance is coin- 

 cident 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 gray matter. In fact, between the ante- 

 rior pyramids on the ventral side and the largely increased and laterally ex- 

 panded gray 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 imbedded in 

 neuroglia. Though differing from the ordinary gray matter of the cord by 

 the more open character of its network, it may be considered as a form of 

 gray matter. We may consider it as being in reality the gray 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 518, which since they curve through this area from the 

 middle line laterally are called arcuate or arciform fibres, internal arcuate 

 fibres (Fig. 132, 6, /. a. i.) to distinguish them from the external arcuate 

 fibres (/. a. e.) of which we shall speak presently, Fragments of more 

 compact gray matter also belonging probably to the anterior horn are seen 

 at intervals in this area (Fig. 132, 6, ac.) and elsewhere. We have seen that 



