MODE OF ORIGIN OF NERVE FIBRES IN NERVE CENTRES. 17") 



part concealed by the presence of a considerable quantity of 

 the interfibrillar substance. And just as the medullary sheath 

 is not essential to our conception of a nerve fibre, so we can 

 only regard it as forming an accessory sheath to the ganglion 

 cell, to which, indeed, it rarely constitutes a continuous invest- 

 ment. The sheath of Schwann, if present, is continued over 

 the ganglion cell, and forms the above-mentioned capsule of 

 nucleated connective tissue. It is, however, absent in the 

 bipolar ganglion cells of the auditory nerve. 



The structure of the spinal ganglia of other vertebrata and 

 of man is more complex. It has been frequently observed, and 

 has very recently been corroborated by the researches of 

 Schwalbe,* that the cells of these ganglia each possesses for the 

 most part only a single non-medullated process which runs 

 towards the periphery, and which, according to Kolliker, subse- 

 quently becomes the axis cylinder of a medullated nerve fibre. 

 Like the substance of the ganglion cells, it presents a fibrillated 

 structure. From some of the cells, on the other hand, instead 

 of a single process, several are given off, which, however, do 

 not arise, as in fishes, from the opposite poles of the cells, and 

 with the further course of which we are still unacquainted. 

 Observations similar to these were made by Kolliker on the 

 cells of the Gasserian ganglion.")* 



Like those of the spinal ganglia, the cells of the sympathetic 

 ganglia are invested by dense connective tissue, and each pos- 

 sesses a proper nucleated capsule, proceeding from and con- 

 tinuous with the sheath of Schwann, covering the nerve fibres 

 with which it is in connection. The number of these last 

 here also varies to a considerable extent. In the sympathetic 

 of the frog, which has been most frequently examined, there 

 occur, besides such unipolar cells as have just been described, 

 others from which two processes spring in close proximity, of 

 which one winds spirally round the other. The minuter details 

 respecting the mode of connection of these spiral fibres, which 

 were first described by L. BealeJ with the ganglion cells, is still 



* A rchivfilr Mikroskopische Anatomie, Band iv., p. 45. 



t Handbucli der Gewebelehre, 5. Auflage, p. 319. 



I Philosophical Transactions, 1863, Vol. cliii., p. 539. 



