534 



THE NEKVOUS SYSTEM. 



and, lastly, the fibres of the cervical roots displace the thoracic fibres. The difference 

 between the fasciculus gracilis and the fasciculus cuneatus consists simply in this, that 

 the former is composed of the fibres of posterior nerve-roots which have entered the 

 medulla at a lower level than those which enter into the formation of the fasciculus 

 cuneatus. The fibres of the fasciculus gracilis, taking them as a whole, must therefore 

 necessarily run a very much longer course. 



Our kiiQwledge of the constitution of the posterior columns of the spinal medulla is 

 derived largely from the study of the course of degeneration in monkeys, after the medulla 

 has been cut across either partially or completely. But we have also a direct knowledge 

 of the lamination of the posterior columns of the human spinal medulla (Fig. 473) that 

 has been acquired from the examination of cases in which the medulla or its nerve-roots . 

 had been injured during life. 



Numerous collateral fibrils stream into the gray matter of the posterior column 

 both from the ascending and descending branches of the entering fibres of the 

 posterior nerve-roots. These are classified into long and short collaterals. The long 



Fasciculus gracili 



Fasciculus septomarginalis 

 Fasciculus gracilis 



Fasciculus cuneatus 



Fasciculus posterolateralis- 



Fasciculus spino- 

 cerebellari^ (posterior)- 



Fasciculus 

 interfascicularis _ 

 (comma tract) 



Fasciculus posterior 

 fcproprius" 



Fasciculus later- 

 alis proprius 



Fasciculus 



anterolateralis 



superficialis' 



(Gowers) 



Fasciculus 



jspinothalamicus' 'A'/-; 

 (posterior) 



Fasciculus/ 

 .spinotectalis 



' Fasciculus 

 anterior proprius 



I 



Fasciculus spinothalamicus anterior | 



Fasciculus cerebrospinalis anterior 



Fasck s 



Fascii 



spinal is (< 



olivospina 



^.^ Fasciculus 

 "" ^ vestibulospinalis 



ANTERIOR NERVE ROOT 

 Fasciculus vestibulospinalis 



Area sulcomarginalis 



FIG. 473. A DIAGRAM TO ILLUSTRATE THE GROUPING OF THE VARIOUS FASCICULI IN THE SPINAL 



MEDULLA (in transverse section). 



collaterals extend forwards into the anterior column of gray matter and end in relatio 

 to the ventral nerve-cells. The short collaterals end in relation to the nerve-cells in th 

 substantia gelatinosa, and other nerve-cells of the posterior column of gray matter. 



The majority of the fibres of the posterior nerve-root enter the spinal medulla o 

 the medial side of the apex of the posterior column of gray matter. The manner in whic 

 these are related to the fasciculus cuneatus and the fasciculus gracilis has been noticed ; bu 

 a certain number of those fibres which lie most laterally take a curved course forward 

 on the medial side of the posterior column of gray matter and then pass into it. I 

 the thoracic region these curved fibres end in connexion with the cells of the nuclei 

 dorsalis (Fig. 467, B, p. 525, and Fig. 473). 



Fasciculus Posterolateralis (O.T. Tract of Lissauer). The postero-laten 

 fasciculus is a small tract of nerve-fibres of minute calibre which assume their medullar 

 sheaths at a comparatively late period. It is placed at the surface of the medulla clos 

 to the sulcus lateralis posterior. It is formed by some of the lateral fibres of the posteric 

 nerve-roots which do not enter the fasciculus cuneatus, but pass upwards in the medul 

 close to the substantia gelatinosa, in which they ultimately end. 



It must now be evident that the fibres which enter the medulla spinalis through eac 



