DEVELOPMENT OF ^^ERVES. IGl 



with a ganglion, and a motor root, like the spinal nerves. The glosso- 

 pharyngeal and pneumo-gastric nerves are also decidedly compound in 

 nature ; they are also provided with ganglia at their roots, which in- 

 volve a greater or less number of their fasciculi ; but it has not yet 

 been satisfactorily determined whether in these nerves the fibres which 

 have different properties are collected at the roots into separate bundles, 

 nor how they are respectively related to the ganglia. The sympathetic, 

 as already stated, contains both afferent and efferent fibres. 



Simple nerves are such as contain either afferent or efferent fibres 

 only. The olfactory, auditory, and optic are simple afferent and sen- 

 sory nerves. The third, fourth, and sixth, the facial, the spinal accessory 

 and hypoglossal nerves are generally regarded as examples of simple 

 motor nerves ; there is reason to believe, at least, that they are simple 

 and motor in their origin, or as far as their proper fibres are concerned, 

 and that the sensibility evinced by some of them in their branches is 

 owing to sensory fibres derived from other nerves which join them in 

 their progress. 



The nerves governing the motions of the blood-vessels are commonly 

 spoken of as the " vaso-motorial nerves ; " but although this term is 

 often of convenient application, there seems no sufficient reason for 

 reckoning these nerves as a distinct system, any more than motorial 

 nerves distributed to other parts or organs whose motions are inde- 

 pendent of the will. 



DEVEIiOPMENT OP NERVES. 



The knowledge as yet acquired respecting this process is not very 

 positive or consistent, so that much room is left for speculation and 

 conjecture. The nerve-cells are doubtless derived from the common 

 embyro-cells, which, undergoing modification in their substance, send 

 out branches fi'om their circumference and acquire the character of 

 nerve-cells. According to the most generally current descriptions, the 

 fibres are stated to be formed by the linear coalescence of long fusiform 

 cells, and to be at first pale and grey, but afterwards to accjuire a medul- 

 lary sheath and become white. This change of aspect is apparent in 

 the human embryo of the fourth or fifth month. According to KiJlliker's 

 account of the growth of nerve-fibres at their peripheral ends, as observed 

 in the tail of batrachian larva3, the existing fibres are prolonged by lines 

 of fusiform cells which coalesce into pale fibres. These send out fine 

 offshoots, which may join with neighbouring fibres, or with branched 

 or stellate cells, which change into branched fibres, and in both of these 

 ways the branching and conjunction of the nerves go on. The first 

 fibres thus generated (embryonal fibres, Kiill.) virtually represent 

 bundles of two, three, or more tubular dark-bordered fibres, into which 

 they are speedily converted ; the formation of the medullary sheath 

 proceeding outwards along the branches. 



The fact pointed out by Eanvier that the medullary sheath of the 

 nerves is divided at regular intervals into a series of segments, each of 

 which possesses a nucleus, and may therefore be looked upon as repre- 

 senting a cell, the primitive sheath being analogous to the cell mem- 

 brane, would seem to render it probable that these segments are actually 

 formed from cells, which come to be applied around previously -formed 

 axial fibres, and become filled with the fatty matter of the medullary 



