STRUCTURE OF THE MEDULLA OBLONGATA. 



613 



been shown by Mr. Lockhart Clarke to be traceable on either side through 

 the thin lamina? constituting the Valve of Vieussens, and through the 

 columns forming the lateral boundaries of the Aqueduct of Sylvius, to near 

 the floor of the fourth ventricle ; whilst the third nerve, the apparent origin 

 of which is from the locus niger of the Cerebral peduncles, has been followed 

 by Stilling to a gray nucleus underlying the Aqueduct of Sylvius, and has 

 been shown by Clarke to be in close relation with the gray tubercle into 

 which the sensory root of the fifth penetrates, and the nucleus common to 

 the portio dura and abducens nerves. In order to complete the description 

 of the origins of the Cerebral nerves, it is only requisite here to mention 

 that the second pair, or Optic nerves, arise from the posterior part of the 

 Optic thalami and from the Corpora Quadrigemina ; whilst the first pair, 

 or Olfactory nerves, arise by three roots, the outer one appearing to be con- 

 nected with the Corpus Striatum of its own side, the inner one with the Lam- 

 ina Cinerea in front of the Optic commissure, and the middle one spring- 

 ing from a gray nucleus in front of the anterior perforated space. It will 

 thus be seen that the whole series of the Cephalic nerves, when followed to 

 their origin, arise from gray ganglionic centres situated along the floor of 

 or just below the fourth ventricle and the base of the brain. These, there 

 can be little doubt, are more or less intimately connected with one another 

 by longitudinal and transverse commissural fibres, and establish the exist- 



FIG. 224. 



Course of the Sensory tract according to Sir C. Bell: A, Pons Varolii ; B, B, sensory tract separated; C, 

 union of posterior columns; D, D, posterior roots of spinal nerves; E, sensory roots of fifth pair. 



ence of a Sensory tract, the activity of which is of fundamental importance, 

 as will hereafter be shown, in the origination of ideas and of a certain class 

 of movements which aiay be termed "consensual " or "seusori-motor." The 

 Pous is chiefly composed of transverse fibres which constitute the great com- 

 missure of the Cerebellum ; and are an extension of the arciform fibres of 

 the medulla oblougata ; and these fibres not only surround the longitudinal 



