48 THE STRUCTURE OF 



cells of the sympathetic, like those last described, are 

 enveloped by a fine membranous sheath dotted with many 

 nuclei, and the sheath is continued for some distance 

 along each of the nerve processes, in the form of a loose 

 envelope. 



8. Lastly, unipolar nerve cells that is, nerve cells 

 situated at the end of a single nerve fibre are alleged to 

 exist in the ganglia on the spinal nerves and elsewhere. 

 They have again recently been figured by Axel Key and 

 Retzius, though many modern observers have been very 

 sceptical as to the existence of such bodies. Beale, for 

 instance, maintains that all nerve cells have at least two 

 processes. Without attempting to explain their use or 

 mode of action, it seems to the writer that such unipolar 

 nerve cells certainly exist in some of the lower animals. 

 He has himself seen and figured such bodies as they occur 

 in Ascaris (Phil. Trans. 1866, PI. xxiv.) ; and in many 

 other animals nerve units of the same kind have been 

 likewise recognized by competent observers. 



Many of the so called apolar nerve cells may, as 

 G. H. Lewes suggests in a recent work,* be nothing 

 more than imperfectly developed ganglion cells, in which 

 the processes, if not absent, are so abortive as to escape 

 observation. All who have examined nerve centres with the 

 microscope know that multitudes of such bodies are to be 

 found, though they are often very small not much larger 

 than mere nuclei and therefore liable to be regarded as 

 belonging to the neuroglia rather than to the nervous 

 tissue proper. And if some of the cells and nuclei usually 

 assigned to the ' neuroglia ' are, in reality, potential or 

 embryo nerve cells, the importance of this intermediate 

 tissue as a formative matrix in which new developments 

 may take place, will at once appear. 



* " The Physical Basis of Mind," 1877, p. 234 



