HITMAN AND MAMMALIAN. 9 



The anterior portion of the gray substance has become more compact, the little 

 wings, noticed in the sections just below, at the entrance of the hypoglossal roots, 

 have nearly disappeared, and the entire substance of the anterior cornua is fiUed 

 with large multipolar cells, constituting the nucleus of the hypoglossal. 



On comparing Figs. 19, 19", from the human medulla, with Figs. 2, 2", from 

 about the same region in the sheep, the dissimilarity seems at first sight consider- 

 able. By a closer examination it will, however, be seen that the general plan is 

 quite the same, and we may consider the form of the human medulla, as resulting 

 from a greater concentration of parts around the central canal, together with the 

 much greater development of some portions, especially the pyramidal and olivary 

 columns. Here, as elsewhere, we see that if we should take the medulla of-the sheep 

 on each side the middle line, near the point where the restiform nuclei approach the 

 posterior surface, and bring the two points towards each other, almost the same dis- 

 position of parts would be produced as is seen in the human medulla, making 

 allowance only for the greater development of certain cell tracts in the latter case, 

 as contrasted Avith the evident simplification of structural details in the sheep and 

 other of the lower mammalia. 



(6.) As we approach the opening of the central canal into the fourth ventricle, 

 the development of the cell groups seen below continually advances, till we reach 

 the level of the calamus scriptorius (Figs. 20, 20"). 



Just in front of the sulcus forming the apex of the fourth ventricle (Fig. 20", w) , 

 and on each side of the middle line, are seen the two tracts closely crowded with 

 large multipolar cells, constituting the hypoglossal nuclei [H) , from which the root- 

 bundles of the hypoglossal {XII) may now be seen radiating to the surface of the 

 medulla, between the pyramidal and olivary columns. The whole anterior and 

 antero-lateral substance contains numerous scattered cells and small cell groups, en- 

 tering everywhere into connection with the arciform fibres ; the extremely compli- 

 cated arrangement of the fibres constituting this plexus has already been very 

 accurately described by Clarke (1858). 



Little groups of cells are frequently found just at the entrance of the hypoglossal 

 roots into their nuclei, as also scattered along the raphe, serving to unite the dif- 

 ferent sets of fibres, transverse, arciform, and longitudinal. 



The olivary convolutions ( 0) have attained a very considerable development, and 

 the small pyramidal nuclei (i) are likewise quite conspicuous. The antero-lateral 

 nuclei [B) have reached their maximum development, being separated into smaller 

 cell groups a little higher up. The caput ib) is large and distinct, and is 

 closely united to the restiform nucleus; they are both crowded with cells, and are 

 comiected with the external band of arciform and oblique fibres (a) by quite nume- 

 rous detached cell groups. At about this height the post-pyramidal and antero- 

 lateral nuclei seem to have reached their greatest development ; the commencement 

 of the auditory ganglion, which is formed out of the substance of the first-named 

 nucleus, is seen in sections lying just above, while the antero-lateral nucleus is con- 

 tinually encroached upon by the development of the olivary bodies. 



(6.) In the next sections (Plates VI and XV, Figs. 21, 21") we have a stiU greater 

 development of some parts, with a corresponding diminution in others. 



