996 COMMISSURAL FIBRES. [BOOK m. 



well as from that of the lateral hemispheres ; they thus form the 

 greater part of the central white matter. Sweeping down into 

 the pons, they form the transverse fibres of that body, interlacing 

 with the longitudinal fibres of the crural system and intermingling 

 with the abundant grey matter. 



Of these transverse fibres of the pons, a certain number are 

 truly commissural; they make no connections with cells in the 

 pons, but continue their way unbroken across it ; they start in the 

 superficial grey matter of one side of the cerebellum and end in 

 the superficial grey matter of the other side, the parts of the grey 

 matter thus united being probably corresponding parts. The 

 most ventrally placed transverse fibres of the pons, which form a 

 superficial layer of white matter, free from grey matter (Fig. 111. 

 tr. P.) are probably of this nature, as are also the transverse fibres 

 placed most dorsally, just ventral to the tegmental region. 



A large number of the transverse fibres are not of this nature. 

 They cross from one side of the cerebellum to the opposite side of 

 the pons, but end in the pons apparently in the nerve cells of the 

 grey matter ; and it is supposed, that by these nerve cells they 

 are brought into connection with the longitudinal fibres of the 

 pedal system and thus with the cerebrum. They are transverse 

 appendages of the pedal system, not true commissural fibres 

 though they do cross the median line. 



It is further supposed that other fibres of the middle peduncle 

 reaching the pons do not cross the median line, but keeping to 

 the same side and changing their direction, take a longitudinal 

 upward course either with or without the intervention of nerve 

 cells, and so make their way to the tegmentum. But this is not 

 certain. 



We must also consider as commissural structures the numerous 

 fibres crossing, or serving to form the median raphe in the bulb. 

 This raphe, with similar commissural fibres, is present in the teg- 

 mental portion of the pons, and indeed in the tegmentum itself. 



Fibres also cross from one side to the other in connection with 

 the cranial nerves, but these as well as all the tracts specially 

 connected with the cranial nerves, including the olfactory and 

 optic nerves, had better be considered by themselves. 



Summary. 



636. It may perhaps appear from the foregoing that the 

 brain consists of a number of isolated masses of grey matter, some 

 large, some small, connected together by a multitude of ties of 

 white matter arranged in perplexing intricacy ; and the addition, 

 of numerous collections of grey matter and strands of white matter 

 of which we have made no mention would still further increase the 

 perplexity. Nevertheless a systematic arrangement may be recog- 

 nized, at least to a certain extent. 



