in6 



HUMAN ANATOMY. 



FIG. 964. 



Superior collicul 



y part) 



extensive of such nuclei, as those within the column of substantia gelatinosa accom- 

 panying the spinal root of the trigeminus, numerous arcuate fibres sweep towards the 

 raphe and, with few exceptions, cross to join the median fillet of the opposite side. 

 In this manner provision is made for the transmission to the higher receptive centres 

 of sensory impulses collected not only by the strands of the posterior column of the 

 cord, but also by the sensory fibres of the cranial nerves attached to the brain-stem. 

 Although the principal components of the fillet-tract are the bulbo-tecto- 

 thalamic strands, some fibres running in the opposite direction are also present. 

 Some of these probably arise from cells within the optic thalamus and the corpora 

 quadrigemina. Others are efferent strands which establish connections between 



the cortical gray matter and 

 the nuclei of the motor cranial 

 nerves, especially the facial and 

 hypoglossal. These cortico- 

 bulbar tracts descend within 

 the crusta to the lower end of 

 the cerebral peduncle ; then, 

 leaving the latter, they traverse 

 the stratum intermedium and in 

 the upper part of the pons join 

 the median fillet and descend 

 within its ventro - median 

 part as far as the superior end 

 of the hypoglossal nucleus. 

 During their course, the fibres 

 of this crustal fillet, as it is 

 called, for the most part undergo 

 decussation on reaching the 

 levels of the motor nucleus for 

 which they are destined ; some 

 fibres, however, possibly end 

 around the cells of the nucleus of 

 the same side. 



The Posterior Longi- 

 tudinal Fasciculus. This 

 bundle (fasciculus loogitttdinalia 

 rncdialis) is an association path 

 of fundamental importance, be- 

 ing present in all vertebrates. As 

 a distinct strand it begins in the 

 superior part of the mid-brain 

 and thence is traceable as a con- 

 tinuous tract through the teg- 

 mental region of the pons, the dorsal and lateral ventral field of the medulla into the 

 anterior ground-bundle of the spinal cord. Throughout the greater part of its course 

 through the brain-stem, its position is constant, the fasciculi of the two sides lying 

 close to the median raphe and immediately beneath the gray matter flooring the Sylvian 

 aqueduct and the fourth ventricle (Figs. 959, 961). In the lower part of the medulla, 

 the bundle gradually leaves the ventricular floor and rests upon the dorsal border of 

 the median fillet, and, at the level of the pyramidal decussation, where the fillet no 

 longer intervenes, lies behind the pyramid and at some distance from the mid-line. 

 Lower, it assumes a more ventral position, to the medial side of the isolated anterior 

 cornu, and, finally, enters the anterior column of the cord to be lost within the upper 

 part of the ground bundle. 



The fasciculus includes association fibres of varying lengths, some of which are 

 ascending and others descending paths. The constitution of the bundle is, there- 

 fore, continually changing, the loss of certain fibres being replaced by the addition 

 of others. Its tibn-s arc among the very first in the brain to become medullated, and 

 begin to acquire this coat during the fourth foetal month (Hosel). 



Sensory decussation 



Posterior nuclei 



Spino-thalamic 

 Spinal ganglion 



Diagram showing chief afferent constituents of median fillet. 



