THE WHITE MATTEE OF THE SPINAL MEDULLA. 533 



!-ovided, therefore, before this is put under the control and direction of the higher centres. It 

 : T no means follows that in all the higher animals corresponding strands myelinate at relatively 

 rresponding periods. Take the case of a young animal which from the time of its birth is able 



move about and perform voluntary movements of various kinds in a more or less perfect 



anner, and compare it with the helpless new-born infant which is capable of exhibiting auto- 



atic movements only. In the former, the cerebro-spinal tracts, or motor tracts, which descend 



i om the cerebrum into the spinal medulla, and which are the paths along which the mandates 



' the will travel, myelinate at an early period ; whilst in the infant the corresponding fibres 



) not obtain their medullary sheaths until after birth. The study of the dates, therefore, at 



hich the various strands of nerve-fibres myelinate not only gives the anatomist a means of 



mating their position in the white matter of the central nervous system, but it also affords the 



Physiologist most important information regarding their functions, and also the periods at which 



lese functions are called into play. 



It is a matter of interest to note that influences which either accelerate or retard the periods 

 . b which nerve-fibres are brought into functional activity have also an effect in determining the 

 ates at which these fibres assume their sheaths of myelin. Thus, when a child is prematurely 

 3rn the whole process of myelinisation is, as it were, hurried up ; and further, when in new- 

 orn animals light is freely admitted to one eye whilst it is carefully excluded from the other, 

 , le fibres of the optic nerve of the former myelinate more rapidly than those of the opposite nerve. 



Study of the minute structure (Anatomical method) of the central nervous system, especially 

 f material that has been stained by the methods of Golgi and Ramon y Cajal or by the use of 

 lethylene blue, completes the results attained by these other methods, by demonstrating the 

 recise mode of origin and termination of the various fasciculi. 



Posterior Funiculus and the Posterior Roots of the Spinal Nerves. In 

 he cervical and upper thoracic regions of the spinal medulla the posterior 

 uniculus is divided by the posterior intermediate sulcus and septum into the 

 asciculus cuneatus, which lies laterally and next to the posterior column of gray 

 natter, and the fasciculus gracilis, which lies medially and next to the posterior- 

 nedian septum. The fasciculus cuneatus is composed of nerve-fibres which are for 

 'he most part larger than those entering into the formation of the fasciculus gracilis, 

 md both tracts have a most intimate relation to the posterior nerve -roots; indeed, 

 /hey are both composed almost entirely of fibres which enter the medulla by these 

 .oo ts, and then pursue a longitudinal course. 



The nerve-fibres which form the posterior nerve-roots, on entering the medulla along the 

 wlcus lateralis posterior, divide within the fasciculus cuneatus into ascending and descend- 

 ,ng branches which diverge abruptly as they pass respectively upwards and downwards. 

 The descending fibres are, as a rule, short, and soon end in the gray matter of the spinal 

 medulla. These descending fibres occupy an area in the posterior funiculus along the 

 line of separation of the fasciculus gracilis and the fasciculus cuneatus, and, hence, may 

 be called the fasciculus interfascicularis (comma tract of Schultze). This area, when 

 the spinal medulla is divided, undergoes descending degeneration and then presents a 

 comma-shaped outline. (Fig. 473). 



The ascending fibres vary greatly in length, and at differing distances from the point 

 where the parent fibres enter the medulla they end in the gray matter. A small contribu- 

 tion, however, of ascending fibres, from each posterior nerve-root, extends upwards to the 

 upper end of the spinal medulla, to end in the medulla oblongata (Figs. 474 and 475). 



As each posterior nerve-root enters, its fibres range themselves in the lateral part of 

 the posterior funiculus close up against the posterior column of gray matter. The nerve- 

 fibres of the nerve-root next above take the same position, and consequently those which 

 entered from the nerve immediately below are displaced medially, and come to lie in the 

 posterior funiculus nearer to the median plane. This process goes on as each nerve- 

 root enters, and the result is that the fibres of the lower nerves are gradually pushed 

 learer and nearer to the posterior median septum in a successive series of lamellar tracts, 

 f course, the greater proportion of the fibres which are thus carried upwards from 

 the posterior nerve-roots sooner or later leave the posterior funiculus and enter the gray 

 matter, to end there in relation to some of its cells ; but, as we have said, every posterior 

 nerve-root sends a few fibres up the whole length of that portion of the spinal medulla 

 which lies above, and thus the posterior funiculus gradually increases in bulk as it is traced 

 upwards, and in all except the lowest part of the spinal medulla, the posterior funiculus 



separable into a fasciculus gracilis and a fasciculus cuneatus. The fasciculus gracilis 



composed of the long ascending fibres of the posterior nerve-roots, which have entered 

 Dwer segments of the spinal medulla. To put the matter differently, the fibres 



the sacral roots are displaced medially by the entering lumbar fibres, while the fibres 



the lumbar roots are 4n their turn pushed medially by the entering thoracic fibres, 



35 a 



