1888.] Nervous System in Vertebrated Animals. 



421 



through it ; the internal layer contains cells mostly rounded. At the 

 central line on the dorsal surface there is a ganglion of large cells 

 resembling those of the optic lobe of the Plagiostomata. 



The cerebellum is a mere bridge over the fourth ventricle, and its 

 structure presents the usual number of layers ; internally there is 

 the fibrous layer which ultimately goes to form the crura cerebelli ad 

 medullam ; then the granular layer, the cells of which are of large 

 size compared to those of the same layer in Teleostei and Plagiosto- 

 mata ; then comes a layer of Purkinje cells, the form and number of 

 processes of which are not uniform ; the external layer is the molecu- 

 lar, which consists of a coarsely granular network derived from the 

 processes of the Purkinje cells, also a network of finer fibrils and 

 many rounded cells. 



In the spinal cord there are three columns of longitudinal fibres on 

 each side in the white substance, viz., the ventral columns between 

 the two ventral roots of the spinal nerves, the lateral columns between 

 the dorsal and ventral roots, and the dorsal columns between the two 

 dorsal roots; fibres of large size are scattered throughout the two 

 former columns, but are collected principally in the ventral ; the dorsal 

 columns consist entirely of fibres of minute size. 



The principal feature in the white substance is a fibre of gigantic 

 dimensions which is situated on the summit of the ventral columns 

 — one on each side ; it consists of a common medullary sheath enclosing, 

 where the fibre is largest, about forty to fifty axis-cylinders ; these 

 have the characteristics of the axis-cylinders of the ordinary fibres of 

 the white substance* but have no separate medullary sheaths; this 

 fibre is traceable throughout the spinal cord ; commencing opposite 

 the posterior end of the abdomen, it extends forward to a short dis- 

 tance behind the exit of the facial nerve ; it varies in size and becomes 

 of the greatest diameter near the posterior end of the medulla oblon- 

 gata; its axes escape through the medullary sheath and join the 

 longitudinal fibres of the ventral columns ; near its anterior termina- 

 tion all the axes have escaped except one ; at this point it bears a 

 great resemblance to Mauthner's fibre in the Teleostei. This re- 

 maining fibre decussates with that of the other side a short distance 

 behind the exit of the facial nerve and joins the root of that 

 nerve. 



In the grey substance of the spinal cord there* are two series of 

 ganglia — one in the ventral horn, which consists of multipolar cells 

 often of very large size ; they send processes into the ventral and 

 lateral columns which often become the smaller-sized longitudinal 

 fibres. The cells of the other series are of smaller size and are 

 situated in the substantia gelatinosa centralis ; they are smooth in 

 outline and give off one or two processes ; they probably have to do 

 with the dorsal roots of the spinal nerves. Cells also of this kind 



VOL. XLIII. 2 H 



