488 THE NERVOUS SYSTEM. 
connexion of the fasciculus solitarius with the medulla is very loose, and it may be 
regarded as being the equivalent in this part of the neural tube of the oval bundle 
of “longitudinal fibres which, in the early cord, constitutes the first stage of the column 
of Burdach. Throughout the further stages of development the fasciculus solitarius 
indicates in a sufficiently clear manner the point of junction between the alar and 
basal laminze. Very soon it becomes covered over by parts developed on its ventral 
aspect, and it ultimately comes to lie deeply in the substance of the medulla. This 
change in the position of the fasciculus solitarius with reference to the surface is asso- 
ciated with a striking developmental process which leads to certain remarkable results, 
and which is termed the formation of the rhombic lip of His (Fig. 361, B and C). Before 
the alar lamina falls outwards, while it still stands erect and its inner surface faces the 
corresponding surface of the opposite lamina, its dorsal edge is folded outwards and 
becomes fused with the outer surface of the remaining portion of the alar lamina. ‘This 
is the rhombic lip, and, when the fusion is complete, a multitude of neuroblasts take form 
within it and migrate in a forward and inwards direction into the ventral parts of the 
alar and basal laminee. The mid-ventral lamina—which consists of spongioblastic cells 
alone and which forms a narrow partition between the two bas is reached on 
either side by the axons of many of these migrating cells. Whilst acting as an impassable 
barrier to the neuroblasts, this spongioblastic septum gives free passage from one side to 
the other to their axons, and a decussation of arcuate fibres in the mesial plane results. 
In this way the raphe of the medulla is formed. The process is very similar to that which 
takes place in the course of the formation of the anterior commissure of the cord, of 
which the raphe may be regarded as the equivalent in the medulla. 
The development of the inferior olivary nucleus and of its two accessory parts is like- 
wise closely connected with the migration of the neuroblasts from the region of the 
rhombic lip. Many of these cells collect together so as to form a nuclear lamella, which 
afterwards assumes its characteristic cr umpled form. 
As the neuroblasts of the rhombic lip stream inwards they pass both on the dorsal 
and the ventral aspects of the fasciculus solitarius, which thus comes to be covered over 
and separated from the surface. The spinal root of the trigeminal nerve, like the 
fasciculus solitarius, is also, in the first instance, throughout its entire course on the 
surface of the medulla, and its change of position in the greater part of its course within 
the pons and medulla is due to the subsequent development of those parts which cover it 
over. 
The importance of the rhombic lip in the development of the medulla will be better 
appreciated if we enumerate the parts which spring from it: (1) the inferior olivary nuclei ; 
(2) the cuneate nucleus ;' (3) the substantia gelatinosa Rolandi; (4) the arcuate nucleus ; 
(5) the internal arcuate fibres ; (6) the olivary system of fibres ; (7) the restiform body. 
The pyramidal tracts which come down from the cerebral cortex are late in making 
their appearance in the medulla. ‘The formatio reticularis precedes them in development. 
They appear in the fourth month of foetal life, and as they are developed the antero- 
median furrow between them takes form on the ventral aspect of the medulla. 
His has pointed out that the earliest-formed part of the medulla is the floor of the 
fourth ventricle, and that the other parts, speaking generally, are added in succession as 
we pass towards the surface. ‘The oldest layer of the medulla is the floor of the fourth 
ventricle with its nuclei. It is followed, in the first instance, by the reticular formation, 
and afterwards by the layer containing the olivary and other nuclei. Last of all come 
the pyramids and the outer (superficial) arcuate fibres ” (His). 
Pons Varolii.—The information which we possess at the present moment regarding 
the development of the pons Varolii is somewhat deficient ; but there is little doubt that 
the course pursued is, in general, very similar to that which has been described for the 
medulla. It has been seen to be composed of parts which are in a great measure 
equivalent to those met with in the medulla, the formatio reticularis of the latter 
passing into the tegmental substance in the former, while the pyramids and arcuate nuclei 
and anterior superficial arcuate fibres of the medulla are represented by the large ventral 
part of the pons. Further, as His points out, similar relations between the chronological 
and local succession of layers may be recognised. Thus the primitive position of the 
motor nucleus of the trigeminal nerve, and also of its spinal root, is a superficial one, and 
it is only by a later process of development that the nucleus pontis and the thick layer 
of transverse and longitudinal fibres are formed. > 
Cerebellum.—The following account of the development of the cerebellum is framed 
largely upon the information supplied in an excellent paper on the subject by Dr. Walther 
Kuithan. 
