186 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



impulse passes to and fro along the common stem which joins the cell-body 

 with the two branches [vide Fig. 72), the stem itself having all the characters 

 of a medullated iibre. 



The study of this modification brings with it the following suggestion : 

 It' the single stem in the modified spinal ganglion-cells must by virtue of its 

 development contain a double pathway, it is fair to inquire whether the same 

 may not be true of the other forms of the nerve-cell in which the axone also 

 appears to be single. Among the cortical cells the arrangement of the 

 branches is such that, for aught that is known, the stem of the axone may 

 functionate in the manner suggested, and contain more than one pathway. 



The same arrangement must exist in the case of cells like those repre- 

 sented in Fig. 76, in which the axone arises from the base of a dendrite 



Fig. 76.— Showing the relations between the terminal branches of the dendrites (D) and of the 

 axones (V) of the optic fibres where they come together in the superficial layer of the optic lobe of the 

 chick ; also showing the origin of the axone (N) from a dendrite (van Gehuchten). 



at some distance from the cell-body, and in which nerve-impulses arriving 

 over the dendrites and leaving by the axone must normally follow the por- 

 tion of the cell-branch which is common to both, passing along it first in one 

 direction and then in the other. This last result has been extended by 

 Sherrington, 1 who found that he could produce movements of the hind limb 

 in both monkeys and cats when the cord had been sectioned just below the 

 bulb, and the stimulus was applied to the fibres in the fasciculi graciles at 

 that level. The reaction is explained by the passage of impulses down the 

 dorsal columns (in a direction reverse to the normal), and their distribution 

 by way of the collaterals to the efferent elements located in the ventral horns. 

 Significance of Cell-Branches. — Since the outgoing nerve-impulses are 

 isolated in the axone until they reach the terminal twigs, it follows that the 

 impulses destined to produce an effect beyond the cell limits will do so at the 

 extremities of the branches. This leads to the question how far the posses- 

 sion of branches is necessary to the functional activity of a nerve-cell either 

 for the reception or transmission of an impulse. Since it has been pointed 

 out that the spinal cord of the newt and fish is capable of conducting impulses 

 even before the dendrites are developed, it follows that the transmission of 

 impulses is in some way dependent on the condition of the cell-wall, inde- 

 pendent of cell-branches. This modification may exist at points where there 

 are no branches, or during this early period be a general property of the 

 1 Proceedings of the Iioyal Society, 1897, lxi. 243-246. 



