PAET I. 



THE FORM AND STRUCTURE 



GRAY SUBSTANCE OF THE MEDULLA OBLONGATA, 

 HUMAN AND MAMMALIAN. 



CHAPTER I. 



MORPHOLOGICAL CHANGES IN THE MEDULLA OBLONGATA OF THE SHEEP. 



(1.) The first change in the form of the gray substance as it passes from the cord 

 to the medulla oblongata, consists in a gradual pushing outwards and forwards of 

 the posterior cornua, Avhich are now traversed along the anterior edge of the cajjitt 

 cornu, especially at its junction with the cervix, by very numerous bundles of 

 longitudinal fibres, forming a beautiful network along the lateral border of the 

 gray substance. At the same time the anterior cornua have rapidly diminished in 

 size, being encroached upon by similar longitudinal fasciculi, extending the above 

 mentioned netAVork into the antero-lateral and anterior columns. The network 

 connected with the posterior cornua is traversed by the roots of the spinal acces- 

 sory, whilst that of the anterior cornua is traversed by the upper cervical, and 

 higher up by the hyx^oglossal roots. These changes are well shown in Plate I, Figs. 

 1, 2; Plate XIII, Figs. 1", 2"', the network in the latter figure being further m- 

 creased by numerous arciform fibres derived from the post-pyramidal and restiform 

 nuclei. 



Anteriorly (Plate I, Fig. 1 ; Plate XIIT, Fig. 1", P') the fibres of the pyramids 

 wiU still be noticed decussating to a considerable extent ; but they have already 

 begun to take a direction parallel with the median line. These latter fibres 

 together with some bundles derived from the arciform plexus, partly running along 

 the middle line, and partly decussating at this point, form the first indications of 

 the raphe, which is shovra. completely formed in Figs. 2, 2", R. 



The most important changes, however, occur in the posterior portion of the 

 medulla. In Figs. 1, 1% it wiU be noticed that the posterior median fissure is 

 still persistent, reaching quite to the posterior gray commissure, which latter is 

 much thicker than in the spinal cord. On each side of the fissure, very minute 



( 1 ) 



