670 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



these localities, although they are by no means so numerous. It is inferred 

 that all the fibres which thus degenerate form paths for the afferent impulses. 



The impulses which come in over a dorsal root on one side can therefore 

 find their way cephalad either by the direct continuations of the dorsal root- 

 fibres running in the dorsal column of the same side, or by way of central 

 cells in the lateral column of the same side of the cord, and also to a less 

 degree in the lateral and dorsal columns of the opposite side. 



The tracts which undergo Wallerian degeneration after this treatment 

 include, therefore, those formed by the neurons arising from central cells. 

 These cells have their cell-bodies arranged in a column running the length 

 of the cord. In the neighborhood of this column some of the dorsal root- 

 fibres terminate. In the bulb we are familiar with such groups of cells, well 

 marked as the " nuclei of the sensory nerves," and these cells in the cord, 

 though far less clearly segregated, are the homologues of those in the bulb. 

 If this is granted, then the fibres which are continued from these central cell- 

 groups, whether in the cord or bulb, are also homologous. 



Corroborative of what has been said on the subject of afferent pathways in 

 the cord are the results of Pellizzi. 1 He studied dogs, making use of the 

 method of Marchi, whereby the nerve-sheaths of fibres beginning to degen- 

 erate or the nutrition of which is disturbed give a characteristic reaction ; he 

 found, after hemisection of the cord, the same lesions that have been described 

 above, with the addition that the changes could also be followed in some of 

 the fibres of the ventral roots. More significant, however, is the fact that 

 section of the lumbar and sacral dorsal roots, without direct injury to the 

 cord, gave rise to modifications of the medullary sheaths, detectable by the 

 method of Marchi, in all the localities just named. 



A distinction must be made at this point. Wallerian degeneration in the 

 central system means eventual destruction of the severed fibre. The method 

 of Marchi shows a characteristic change in fibres entering upon this degen- 

 eration, but this method also shows changes in the sheaths of elements which 

 are only physiologically connected with those about to undergo Wallerian 

 degeneration, but which themselves are, as a rule, not ultimately destroyed. 

 Under the usual conditions of experiment Wallerian degeneration is confined 

 within the morphological limits of a single cell-element, but the physiological 

 changes in the cells overstep this limit, as shown by Marchi's reaction. It is 

 proper to add, also, that Wallerian degeneration may under some conditions 

 extend to a group of nerve-cells only physiologically connected with those 

 suffering the initial injury. 



Physiological Observations on Afferent Pathways. Making use of 

 the fact that strong stimulation of the sensory fibres, such as those in the 

 sciatic nerve, causes a rise in blood-pressure, Woroschiloff 2 sought to block 

 the passage of the impulses causing this reaction by section of the cord in 

 different ways in the upper lumbar region of the rabbit. It appears that in 



1 Archives Italiennes de Biologie, 1895, Bd. xxiv. 



2 Berichte der math.-phys. Classe d. k. Gesellsch. d. Wissen. zu Leipzig, 1874. 



