CENTRAL NERVOUS SYSTEM IN' VERTEBRATE ANIMALS. 
947 
oblongata. At the point of their crossing, the Mauthner fibres present the appearance 
as if the axis cylinder had broken up into fibrillse. The medullary sheaths are entire, 
and in them are seen several blue dots* like sections of fibrillse ; in the next section 
forward both medullary sheaths and blue points have completely disappeared, while in 
the few sections behind this part, the axis cylinder is seen gradually breaking up into 
these fibrillse, the lower one becoming decomposed at a point behind the upper. This 
confirms what I have long suspected : that the axis cylinders of the Mauthner’s fibres 
are really composed of fibrillse, and as the Mauthner are merely gigantic nerve fibres it 
might confirm the views of Max Schultze,! who was of the opinion that ordinary nerve 
axis cylinders are made up of fibrillse, united together into a single fibre. 
The central longitudinal columns disappear at a very short distance in front of the 
decussation above mentioned, and the ventral longitudinal columns take their place 
beneath the floor of the fourth ventricle, at the anterior end of the commissure of the 
tuberculum impar. At about the position of the ganglion of the oculomotorius these 
disappear as compact bundles, but can be traced forward as isolated fibres and small 
fasciculi as far as the region of the posterior commissure to which they appear to 
contribute fibres. 
Transverse Commissures of the Central Nervous System. 
In the spinal cord two transverse commissures are present, corresponding to those 
of M. cejphalus ; they connect the dorsal and ventral horns of grey matter of each side ; 
the former is placed close to the dorsal edge of the central canal and the latter occupies 
its usual place between the ventral and central columns (figs. 13, 14, and 15). 
More anteriorly an immense decussating commissure (fig. 7) is met with which is 
connected with the tuberculum impar, and is developed in like proportions. This is 
evidently the continuation forward of the ventral commissure of the cord; it occupies 
the same level and separates the two ventral columns in the same manner. It exists 
only at the anterior part of the tuberculum where it forms a communication between 
one side of that lobe and the opposite side of the medulla oblongata. Its bundles are 
seen to arise from the complex of fibres of origin of the trifacial in the central portion 
of the tuberculum, then to wind round the central nucleus of finely granular matter 
or sixth layer, to pass beneath what there is of the fourth ventricle and finally to 
pass beneath that ventricle to the other side, where they are lost quite close to the 
lateral and inferior edge of the medulla oblongata. This commissure is traceable in 
the myelencephalon as far forward as the posterior side of the cerebellum. 
The commissura ansulata (fig. 6) occurs further back than in Mugil , being placed 
beneath the cerebellum instead of beneath the middle of the valvula cerebelli. That 
it is the commissure which is displaced backward, rather than that the cerebellum is 
displaced forward, is I think indicated by the fact that the root of the oculomotorius 
* The staining fluid was rosaniline. 
t S thicker’ s Handbook, Sydenham Society. 
