ON THE MAMMALIAN NERVOUS SYSTEM. 365 



erroneous character of this view. The nerve fibres in the cord respond to direct exci 

 tation, like those in the nerve roots and nerve trunk ; that is to say, nerve impulses, 

 with their accompanying electrical effects, are propagated in both directions along 

 any continuous nerve fibres which may exist in the excited area ; but, in addition, 

 complications are introduced by the connection of a large number of the fibres with 

 nerve cells, this connection causing now a possible decrease in the total electrical 

 effects, presumably by the blocking of the path and the falling out of certain 

 impulses, now an increase, presumably by the awakening of cells which lie in the path, 

 and the accession of fresh impulses generated in these structures. 



We will first state in general terras upon what facts rests our present knowledge 

 of the mode of propagation of nerve impulses, from an excited area of the cord along 

 continuous paths to a distant unexcited area. 



(a.) It has been already pointed out in the historical introduction that the 

 methods of histological investigation, particularly those associated with the presence 

 of developmental and degenerative changes, have unravelled from the skein of nerve 

 fibres in the cord certain tracts, and grouped them into columns of a continuous 

 character in the lateral and posterior regions respectively. The limits of our 

 knowledge have already been alluded to, but the inadequate character of the method 

 is shown by the large number of fibres which are displayed in each transverse section 

 of the cord, and the comparatively small number as to which a continuous connection 

 with other parts of the cord has been demonstrated. 



(b.) When we turn to the results of physiological experiments, the only method 

 which has furnished satisfactory indications of direct physiological continuity in a 

 tract of nerve fibres is that employed by WOKOSCHILOFF, SCHIFF, and others, of 

 exciting the peripheral end of the cut cord below the medulla and observing the 

 muscular movements of the lower limbs, in one case with the lower part of the cord 

 intact, in another with a section of some structurally known column. In this way 

 it has been shown that a group of fibres in the lateral column forms a path of such 

 direct continuity between the seat of excitation in the cervical region and the lumbar 

 cord that its section interrupts the passage of the descending nerve impulses gene 

 rated in the former region, and it is therefore inferred that this path is, physiologically 

 speaking, directly continuous. 



(c.) Another method of determining the character of propagation in the spinal paths 

 is the classical method employed by HELMHOL.TZ in the case of the nerve trunks, that 

 is, the measurement of the transmission time. This, in the case of the fibres in the 

 nerve trunk (Frog, Rabbit), is generally held to be about 33 metres per second, 

 though in Man the conduction along sensory fibres is stated to be twice as fast. 

 The experimental determination of this latter is, however, complicated by the 

 methods used, which include the measurement of the individual reaction time, and 

 introduce, therefore, additional uncertain factors which blur the clearness of the 

 results. 



