CENTRAL NERVOUS SYSTEM. 179 



From this mode of development it is plain that the single stem must be 

 looked upon as containing a double pathway, although it appears to be in all 

 ways a single fibre, for on the one hand it contains the path for the incoming 

 and on the other for the outgoing impulses. Recent investigations have shown 

 in a striking way that cells modified in this manner are by no means limited 

 to the spinal ganglia, but occur in the cortex of the cerebellum and elsewhere. 

 Classifying the nerve-cells, therefore, in the light of these facts, we find : 

 (1) The pyramidal type, in which the dendrites and axone are both well 

 developed, and in which the greater number of the impulses most probably 

 enter the cell by way of the dendrites and leave by way of the axone ; (2) The 

 spinal ganglion type, in which originally the impulse passes in at one pole of 

 the cell and out at the other, but in the course of development the twoaxones 

 become attached to the cell-body by a single stem, and by inference there 

 must be in this stem a double pathway. In this latter case there are usually 

 no dendrites. 



Growth of Branches. — After the cells have taken on their type-form the 

 branches still continue to grow, not only in length, but also in diameter. In 

 man, for example, the diameter of the nerve-fibres (axones) taken from the 

 peripheral nerves at birth is 1.2-2 fi for the smallest, up to 7-8 // for the 

 largest, with an average of 3-4 /x, while at maturity it is 10-15 [i for the 

 larger fibres. 1 



Internal Structure of the Neurones. — The status of this problem has 

 been admirably summarized by Barker, 2 to whose book the reader is referred. 

 For our purpose it is sufficient to state that the cytoplasm of nerve-cells is 

 composed of fibrils (the character of which is much discussed), and an inter- 

 mediate, non-fibrillar material. These constituents are distributed in different 

 proportions in the several parts of the neurone. The axone contains the 

 fibrils most closely packed. The intermediate substance is most evident in 

 the body of the cell, and in general the dendrites more closely resemble in 

 their structure the cell body. Part, at least, of the intermediate material 

 forms the " stainable substance" of Nissl, also called "tigroid," which, in its 

 susceptibility to change under disturbed nutritive conditions, acts like a stored 

 food material. But which portion of the cell acts to conduct the nerve im- 

 pulse is not known, and the contention that one or the other of* the compo- 

 nent structures is the conductor of the nerve impulses rests on histological 

 evidence alone. For the present it is sufficient to know that the neurone 

 appears to be conductive in all its gross parts. 



While the axone is growing as a naked axis-cylinder, it is usually slivjitly 

 enlarged at the tip (Cajal), suggesting thai it is specially modified at that 

 point. The nutritive exchange on which the increase of the entire axone 

 depends appears to take place along its whole extent, and no! l" be entirely 

 dependent on material passed from the cell-body into the axone. 



Medullation. — After the production of its several branches, the next step 



1 Westphal : Neurologisches Centralblalt, 1894, No. 2. 

 'Barker: The Nervous System, L 899, pp. 101-114. 



