510 



THE NEKVOUS SYSTEM. 



axon (a) and subsequently develops a bunch of dendritic processes (6). In the 

 case of the axon reasons have already been given for not accepting this view as the 

 whole explanation ; and in the case of the dendrites, although the appearance of 

 microscopic sections seems to favour the view expressed in the diagrams, the fact 

 that the neuroblasts are united into a continuous network or syncytium at an 

 early stage of development (see p. 503) raises the possibility that the dendrites 

 may be formed by the gradually drawing out of the existing bridges as the linked 

 cell-bodies become moved apart. 



The Ganglia of the Sensory Nerves. The cells found in the ganglia of the 

 cerebral nerves and on the posterior or dorsal roots of the spinal nerves have a 

 different origin, and present many points of contrast with neurones in the gray matter 

 of the brain and spinal medulla. As already indicated, the ganglia in question are 

 derived from the neural crest. The cells forming these ganglionic masses are some- 

 what oval in form, and each extremity or pole becomes drawn out into a process, so 

 that the neurones become bipolar. These processes are distinguished as central and 

 peripheral, according to the direction which they take. The central processes 



penetrate the wall of the neural tube. In 

 the region of the spinal medulla they form 

 almost the whole of the fibres which enter 

 into .the composition of the posterior roots of 

 the spinal nerves. In the substance of the 

 cerebro- spinal axis they give off numerous 

 collaterals, and after a course of varying ex- 

 tent they end, after the manner of an axon, 

 in terminal arborisations, which enter into 

 relationships with certain nerve-cells in the 

 cerebro-spinal axis. The peripheral processes 

 proceed along the path of the particular 

 nerve with which they are associated, and 

 they finally reach the skin or other sensory 

 surface. Thus, to take one example : the 

 majority of the fibres which go to the skin 

 break up into fine terminal filaments, which 

 end freely between the epithelial cells of 

 the epidermis. The two processes of a 

 ganglion cell, therefore, form the afferent 

 fibres of the cerebro-spinal nerves, and con- 

 stitute the path along which the influence 

 of peripheral impressions is conducted to- 

 wards the brain and spinal medulla. The 

 body of the cell is, as it were, interposed in the path of such impulses. 



But the original bipolar character of these cells, with very few exceptions 

 (ganglia in connexion with the acoustic nerve and the bipolar nerve-cells in the 

 olfactory mucous membrane), gradually undergoes a change which ultimately leads 

 to their transformation into unipolar cells. This is brought about by the tendency 

 which the cell-body has to grow to one side, viz., the side towards the. surface of the 

 ganglion (v. Lenhossek). This unilateral growth leads to a gradual approxima- 

 tion of the attached ends of the processes, and finally to a condition in which they 

 appear to arise from the extremity of a short common stalk in a T-shaped manner 

 (Fig. 452). It is interesting to note that in fishes the original bipolar condition 

 of these cells is retained throughout life, without change. 



Both" the central and peripheral processes of these ganglionic cells become the 

 axis-cylinders of nerve-fibres, which, acquiring a medullary sheath, belong there- 

 fore to the medulla ted variety. From this it might very naturally be thought that 

 the ganglionic neurone, with its two axons and no typical dendrites, is a nervoue 

 unit very different from a neurone in the gray matter of the cerebro-spinal axis. It 

 is believed by some, however (van Gehuchten and Cajal), that the periphera 

 process, in spite of its enclosure within a medullary sheath, and though presenting 

 all the characters of a true axon, is in reality a dendrite If this is the case, th< 



FIG. 452. THREE STAGES IN THE DEVELOPMENT 

 OF A CELL IN A SPINAL GANGLION. 



