ME. J. L. CLAKKE ON THE INTIAtATE STETTCTUEE OF THE BEAIN. 273 
ceeding transversely outward to the lateral column, and crossing the fibres of that 
column which run to the opposite pyramid. These fibres take no share in the formation 
of the pyramids ; and after running outward for a short distance they turn round and 
descend the cord obliquely across the longitudinal fibres of the lateral column. At the 
points where these two sets of fibres bend round (where the former or transverse set 
descend, and the lateral or longitudinal set become transverse as they cross to the oppo- 
site side) there is a very complicated and curved interlacement of bundles. Now the 
physiological importance of the former set is evident when we consider that the part of 
the posterior grey substance from which they arise is precisely that which forms the 
lower portion of the spinal-accessory and vagal nuclei, and with which the lower roots 
of the spinal-accessory nerves are connected. It is moreover interesting to observe that 
in ascending the medulla oblongata above the decussation of the pyramids, a similar 
system of fibres was found to proceed from the same respiratory centre, and to run 
down the lateral columns. 
(15) Since the publication of my memoir on the medulla oblongata I have ascertained 
some further and important particulars regarding the structure and connexions of the 
respiratory centres. Fig. 25, Plate X. represents with great exactness a transverse 
section of the central canal with the left lateral half of the spinal-accessory and hypo- 
glossal nuclei, at the level of the highest roots of the spinal-accessory nerve, as shown 
on a smaller scale in the entire transverse section of the medulla in fig. 8, Plate VIII. 
The spinal-accessory nucleus consists of two portions, an anterior and a posterior portion, 
which differ from each in structure, although they are closely connected. The poste- 
rior portion (A A, B, D, M, fig. 25, Plate X.) slopes outward and forward and is overlain 
by the posterior pyramid (see b 1 , fig. 8, Plate VIII.). It contains two or three thick 
longitudinal columns of grey substance, which in transverse section appear as oval or 
roundish masses (B, fig. 25, Plate X.), either wholly or only partially separated from 
each other. These masses consist of granules mixed with nuclei and small cells, and 
are enveloped by curved fibres and traversed by others in different directions. Some of 
the curved fibres at the outer borders of these masses run obliquely outwards and down- 
wards along the medulla in straight lines to reach the base of the restiform body. Others 
from the anterior and posterior parts of the same or of different masses proceed inward , 
and, decussating across the middle line, connect these masses with their fellows of the 
opposite side (C C, fig. 25, Plate X.). This decussation of fibres is not observable in every 
section, and is most conspicuous a little lower in the medulla. Another set .of fibres 
proceeding out of these masses run obliquely forward and outward (D) to the lateral 
column, and are joined by numerous others which issue from between the pyramids, at 
the bottom of the median fissure (E), and run on either side of the masses in the same 
direction. Those on the outer side (A A) curve forward and inward, and terminate in a 
plexus of bundles which decussate across the raphe (F) with their fellows in the oppo- 
site half of the medulla. They are interspersed with numerous small cells, of which 
some are darkened by pigment. The fibres on the inner side (G) run forward and 
2 q 2 
