20 



GRAY SUBSTANCE OP THE MEDULLA OBLONGATA. 



cesses by which they are embraced. In Fig. 3, still higher, the group we have been 

 considering appears to be divided ; it is more completely within the limits of the 

 cervix, and consists of two portions, the anterior (a), reached by the posterior roots 

 of the spinal accessory (XI), constituting together with the cells of the tractus 

 intermedio-lateralis, which are successively pushed further inwards, the posterior 

 nucleus of this nerve ; and a posterior group (cZ), which, as will presently be shown, 

 is that from which the post-pyramidal and restiform nuclei are developed. 



In Fig. 4, showing the posterior cornu just below the pyramidal decussation, the 

 continuance of this change is strikingly shown. Here the groups are very distinct, 

 the posterior being separated from the anterior by a broad band of fibres which 



Fig. 4. 



Posterior cornu from the spinal cord of the cat, just below the pyramidal decussation, 

 fig. 2 ; S, posterior spinal nerve root. 



The letters correspond with 



proceed from the spinal accessory towards the posterior commissure. The cells of 

 the tractus intermedio-lateralis have mostly joined the anterior group formed from 

 the posterior vesicular column, and are reached by the posterior roots of the spinal 

 accessory ; a few cells belonging to this tract still lingering among the network along 

 the lateral portion of the cornu are seen at /. The cervix is almost entirely separated 

 from the caput cornu by very numerous longitudinal fasciculi which pierce the gray 

 substance. In the sheep, the cells of the posterior vesicular columns, with the 

 exception of a few scattered cells, seem to disappear in the region of the first cervi- 

 cal nerve, being replaced by a little group of small, oblong cells, the nucleus of the 

 spinal accessory, formed almost exclusively, however, from the tractus intermedio- 

 lateralis, which is here pushed in towards the central canal ; but higher up the cells 

 of the posterior vesicular columns reappear, forming the restiform and post-pyrami- 

 dal nuclei, and uniting with the above-mentioned group in forming the much 



