198 



NINETEENTH LECTURE. 



Our structure is surrounded by an envelope. It appears 

 thick, a sort of nucleated connective tissue at the first glance ; 

 however, the nuclei may have another signification, for, on 

 the inner surface of the capsule, a lining of endothelial cells 

 has subsequently been noticed. 



This envelope appears more simple and thinner around the 

 ganglion cells of the lower vertebrates, fishes (Fig. 175) and 

 amphibia. 



At the first, most cursory examination — and the older his- 

 tologists, with their bad methods of investigation, arrived no 

 further — all the peripheral ganglion cells appear to have no 

 processes or, as a scholastic expression runs, are apolar. We 

 have subsequently adopted an entirely different view ; apolar 

 ganglion cells either do not occur at all, or only exceptionally 



as embryonic, arrested in 

 their development, and 

 possibly futureless ele- 

 ments. 



About 1845, Koelliker, 

 one of the most cele- 

 brated histologists, dis- 

 covered in the sympa- 

 thetic of the vertebrates 

 ganglion bodies which 

 sent off from one of their 

 ends a pale filament, which 

 after a sometimes shorter, 

 sometimes longer course, 

 was enveloped in a me- 

 dullary sheath, and be- 

 came a nerve fibre (Fig. 



175,4 



In vertebrate creatures 



something of the kind 

 has, it is true, been previously seen. These are the so-called 

 unipolar ganglion cells. 



Soon after this, R. Wagner, Robin and Bidder, with Rei- 



Fig. 175. — From the peripheral nerve ganglion of a 

 fish, godus lota ; a, b, bipolar ganglion 'cells ; c, uni- 

 polar ; d, e, abnormal forms. 



