SUMMARY. 635 



internal gray matter, and to those of the inferior peduncle which start 

 chiefly from 'the superficial gray matter of the verrnis, appear to start from 

 the superficial gray matter of the whole surface, from that of the median 

 vermis as 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 longitu- 

 dinal fibres of the crural system and intermingling with the abundant gray 

 matter. 



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

 missural ; they make no connections with cells in the pons, but continue 

 their way unbroken across it ; they start in the superficial gray matter of 

 one side of the cerebellum and end in the superficial gray matter of the other 

 side, the parts of the gray 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 gray matter (Fig. 134, 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 gray 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 teg- 

 mentum. 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 tegmental 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. 



549. It may perhaps appear from the foregoing that the brain consists 

 of a number of isolated masses of gray matter, and some large, some small, 

 connected together by a multitude of ties of white matter arranged in per- 

 plexing intricacy ; and the addition of numerous collections of gray 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 recognized, at least, to a certain extent. 



The least conspicuous, but perhaps in point of origin the oldest part of the 

 brain, seems to be what we have called the central gray matter. This seems 

 to serve chiefly as a bed for the development of the nuclei of the cranial 

 nerves. 



Next to the central gray matter and more or less associated with it comes 

 what is called the tegmental region, of which the reticular formation, com- 

 ing into prominence in the bulb and continued on to the subthalamic region, 

 forms, as it were, the core. Belonging to the tegmental system are numerous 



