General Somatic Afferent System 33 



reticular nucleus of the tegmentum, from which, apparently, 

 new fibres continue the pathway forward with the medial 

 lemniscus to the thalamus. 



Dorso-medial to the chief sensory \^ nucleus, lies a con- 

 spicuous group of large unipolar cells mingled with several 

 small bundles of stout myelinated fibres. The cells give off 

 similar fibres, which join those in the small bundles and run 

 with them into the fifth nerve. Each fibre bifurcates, how- 

 ever, before passing out of the brain, one branch remaining 

 within and ending either in the motor \' nucleus or in a small 

 group of cells dorso-medial to the sensory \' nucleus. If the 

 small bundles be now followed up the brain, they are found 

 to be associated with a continually decreasing number of the 

 unipolar cells, each of w^hich gives rise to one of the fibres in 

 the bundles, scattered along the lateral aspect of the central 

 gray matter up to nearly the anterior end of the midbrain. 

 These fibres form the mesencephalic root of the trigeminus 

 {radix mesencephalica trigemini), the unipolar cells making 

 up its nucleus (Pis. XI. -XIV.). They are sensory in function, 

 probably concerned with muscle sensibility, the unipolar 

 cells being apparently equivalent to peripheral ganglion cells 

 which have developed within the brain instead of outside it. 

 In fact some of the fibres of the mesencephalic root do arise 

 from similar cells in the semilunar ganglion. 



Degeneration experiments (Papez) reveal a group of descending 

 fibres (Tract of Probst) rising apparently in the mesencephalic V root and 

 running back in the rat dorsal to the chief sensory and the motor nuclei 

 of the trigeminus. It continues posteriorly in the dorsal part of the 

 reticular formation just ventro-medial to the solitary tract and its nuclei, 

 and probably ends in relation to the salivatory nuclei. 



Besides those somatic afferent fibres which enter through 

 the cranial nerves, the medulla oblongata receives others 

 which come up from the spinal cord. The fibres of the spinal 

 lemniscus pass up from the white matter of the cord into the 

 reticular formation of the medulla oblongata, whence they 



