THE SENSOKY PATHS 533 



tion of the arcuate fibres already described, and, later, of similar 

 fibres from the cranial nerves. It forms the chief sensory or cen- 

 tripetal pathway, and, as it continues the impulses received from 

 "Path I" of the spinal cord, it contains neurones of the second order, 

 whose cell bodies lie in the nuclei of Groll and Burdach. Neurones 

 of the second and higher orders coming from the sensory nuclei of 

 the cranial nerves join the fillet in the medulla oblongata and mid- 

 brain. 



In the pons Varolii the mesial fillet also forms a flattened ribbon 

 on either side of the median raphe, but gradually assumes a more 

 ventral position, thus becoming separated from the posterior longi- 

 tudinal fasciculus which makes a dorsal curve in its passage through 

 the medulla and pons. 



In the upper portion of the pons the mesial fillet lies in the ven- 

 tral part of the tegmentum, but spreads somewhat lateralward, 

 becoming thus twisted upon itself, so that the long diameter of its 

 sectional area now lies in the transverse axis of the pons instead of 

 being in the ventro-dorsal axis as in the medulla oblongata. 



In the mesencephalon the tracts of the mesial fillet become still 

 more divergent. They now occupy the lateral margin of the teg- 

 mentum where their dorso-lateral margin blends with the newly 

 formed lateral fillets.* 



In the diencephalon the paths of the fillet continue their diverg- 

 ent course. They pass to the optic thalami where many of their 

 fibres have end arborizations. Many of the fillet fibres also have 

 "relay stations," as it were, all along their course e.g., in the 

 formatio reticularis of the medulla oblongata and pons Varolii, and 

 in the grey matter of the hypothalamic region from which they 

 are continued by neurones of higher orders to, or possibly beyond, 

 the thalami. 



From the optic thalami, by neurones of the third or higher 

 orders, the path of the fillet is continued into the posterior limb of 

 the internal capsule just behind the motor tract ; thence they follow 

 the radiations of the corona radiata to the cerebral cortex. 



The tract of the fillet acquires many fibres throughout its ex- 

 tensive course, which, though all centripetal and therefore subject 

 to ascending degeneration, are of very diverse function and con- 

 sequently possess a very broad distribution within the cerebral 

 cortex. 



* The lateral fillet is a portion of the central auditory path which enters the 

 medulla through the cochlear portion of the eighth cranial nerve. 



