124 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



pendent anatomical element. These authors contend that the neurofibrils of 

 the axis cylinder pass through the nerve cells and enter by way of a network 

 into direct connection with the neurofibrils of other neurons (see Fig. 59). 

 The neurofibrils form a continuum through which nerve impulses pass without 

 a break from neuron to neuron. According to this conception, the ganglion 

 cells play no direct part in the conduction of the impulse from one part of the 

 nervous" system to another; the neurofibrils alone, and the intracellular 

 and pericellular networks with which they connect, form the conducting 

 paths that are everywhere in continuity. In the explanation given below 

 of the activities of the nervous system the author, following the usual cus- 

 tom, makes use of the neuron doctrine, since it is at present impossible to 

 say whether or not the newer views of the continuum of neurofibrils will be 

 corroborated. While the physiological facts remain the same whichever view 

 prevails, there can be no doubt that the point of view of the physiologist 

 would be greatly changed if the present simple conception of a series of neu- 

 rons of a definite polarity as regards conduction were replaced by the more 

 complex schema of independent neurofibrils and a central reticulum in which 

 a basis for polarity and definite paths of conduction is lacking. 



The Varieties of Neurons. The neurons differ greatly in 

 size, shape, and internal structure, and it is impossible to classify 

 them with entire success from either a physiological or an anatomical 

 standpoint. Neglecting the unusual forms whose occurrence is 

 limited and whose structure is perhaps incompletely known, there 

 are three distinct types whose form and structure throw some 

 light on their functional significance: 



I. The bipolar cells. This cell is found in the dorsal root gan- 

 glia of the spinal nerves and in the ganglia attached to the sensory 

 fibers of the cranial nerves, the ganglion semilunare (Gasserian) 

 for the fifth cranial, the g. geniculi for the seventh, the g. vestibu- 

 lar and g. spirale for the eighth, the g. superius and g. petrosum 

 for the ninth, the g. jugulare and g. nodosum for the tenth. 



The typical cell of this group is found in the dorsal root ganglia. 

 In the adult the two processes arise as one, so that the cell seems to 

 be unipolar, but at some distance from the cell this process divides 

 in T, one branch passing into the spinal cord via the posterior 

 root, the other entering the spinal nerve as a sensory nerve fiber 

 to be distributed to some sensory surface. Both processes become 

 medullated and form typical nerve fibers. That these apparently 

 unipolar cells are really bipolar is shown not only by this division 

 into two distinct fibers, but also by a study of their development 

 in the embryo. In early embryonic life the two processes arise 

 from different poles of the cell, and later become fused into an ap- 

 parently simple process (Fig. 55). The striking characteristics of 

 this cell, therefore, are that it gives rise to two nerve fibers, and that 

 it possesses no dendritic processes. On the physiological side these 

 cells might be designated as sensory cells, since they appear to be 

 associated always with sensory nerve fibers. So far as the sensory 

 fibers of the spinal and cranial nerves are concerned, it is worth 

 noting also that all of them arise from cells lying outside the main 



