ORGANS OF THE BODY. 581 



557, Z), we encounter smaller cells, in many cases fusiform and of delicate 

 consistence. In them also one process becomes an ordinary, though thin 

 axis cylinder, beside which may be seen again ramifying protoplasm pro- 

 cesses with lateral derivation of the finer axis cylinders of the second 

 order. In size and shape these cells also vary considerably, larger 

 examples resembling in a great measure those of the anterior cornu. 

 These corpuscles of the posterior cornu have been set down as the source 

 of the fibres of the sensitive roots, and been designated as sensory elements, 

 although at present we are possessed of no really complete proof of the 

 correctness of this view: Gerlach reckons them, moreover, among the 

 motor ganglion corpuscles. 



At the base of the posterior cornu, internally, almost throughout the 

 whole length of the cord, other small groups of cells are to be seen (the 

 pillars of Clarke, or nuclei of Stilling, according to Koellilcer). The 

 elements, collected here, are of medium size, round, and ramifying. Very 

 little is as yet known about them. According to Gerlach, further, they 

 are not possessed of an axis cylinder process. Their ramifications merely 

 sink into the dense neural network of the grey substance. 



The proper ganglion cells of the posterior cornu possess, as a rule, in 

 the opinion of the last-named observer, processes only, which merge into 

 that nervous reticulutn just spoken of. From the latter, then, the sensory 

 fibres of the posterior roots arise. 



from all this it would appear that the mode of origin of the motor and 

 sensory nerve fibres is entirely different. 



The delicate neural net-work alluded to is only observed to be absent 

 in the immediate neighbourhood of the axial canal, and in the substantia 

 (jelatinosa of Roland. It may be easily distinguished from the elastic 

 reticulum of the neuroglia by certain reactions according to Gvrlach. 



294. 



Turning now to the posterior roots of the cord (&), we find far greater 

 complication than among the motor bundles of the spinal nerves. Our 

 knowledge, therefore, of the nature of their arrangement is necessarily 

 more scanty than of the latter. The remarkable diminution, besides, in 

 diameter which the sensitive nerve fibres undergo on entering the grey 

 matter, renders the tracing of them very difficult. 



It is stated by some (Koelliker) that the external portion of the pos- 

 terior root-bundles passes directly through the posterior column into the 

 grey substance. Another, and moreover larger part is said, on the other 

 hand, to pursue a rather devious curving course through the hinder 

 column, bending round subsequently in order to pierce from the side the 

 convex border of the posterior cornu, which is turned towards the middle 

 line. From this the fasciculi advance towards the anterior cornu, passing 

 partly into the anterior commissure, and partly in among the posterior 

 group of motor ganglion cells ; or, again, penetrating at times as far as the 

 anterior portion of the lateral column, where they are lost. The first-men- 

 tioned bundles are said to pass forwards, partly as separate longitudinal fasci- 

 culi, tending at the same time with radiation towards the centre, in order to 

 arrive at the pillars of darkens they are called, without becoming connected 

 with cells. Some of them reach the anterior cornua and commissure. 



Commenting on these statements, Deiters showed later that it is 

 always the greater part of the posterior root which takes this curved 

 course through the posterior column, and enters the cornu from it. Here 



