94 



MANUAL OF ELEMENTARY ZOOLOGY 



terminal arborisation, from which a stimulus is given either 

 to another nerve cell or to the elements of some different 

 tissue, as for instance to one of the fibres which compose 

 muscle. The axon is often of very great length, and is 

 then known as a nerve fibre, the term " nerve cell " being 

 applied to its cell body alone. Such a fibre bears at inter- 

 vals nuclei which, however, do not belong to it, but have 

 a small amount of granular cytoplasm of their own. They 

 are called nuclei of the sheath, - because they underlie and 

 appear to belong to a delicate membranous primitive sheath 

 or neurilemma which generally covers the fibre. Nerve 

 fibres are of two kinds, medullated and non-medullated. In 

 the former a layer of fatty substance, known as the medul- 

 lary sheath, lies between the 

 neurilemma and the axon, 

 which is called the axis 

 cylinder. The medullary 

 sheath is broken at intervals 

 at spots known as nodes of 

 Ranvier, and the nuclei are 

 arranged one to each inter- 

 node. In non-medullated 

 fibres the fatty sheath is 

 wanting. Medullated fibres 

 are found principally in the 

 cerebro-spinal system, non- 

 medullated fibres princi- 

 pally in the sympathetic 

 system. 



The grey matter of the 

 central nervous system con- 

 sists of cell bodies embedded 

 in an exceedingly fine network composed of dendrites and 

 terminal arborisations, with fibres passing through it on their 

 way from the cell bodies. The white matter and the nerves 

 consist of fibres only. Ganglia consists of cell bodies inter- 

 spersed with fibres, some or all of which make connection 

 with the cells. In the dorsal root ganglia each cell has only 

 one process, and this after running for a short distance 

 divides at right angles, giving off in one direction an axon 

 which enters the central nervous system, and receiving in 



Fig. 53. — Germs of the frog. A, 

 spermatozoon ; B, unripe ovum 

 in the ovary ; C, ripe ovum. 

 Magnified, but not drawn to 

 scale. 



jl., Flagellum \fol., follicle of germinal 

 epithelium cells around ovum ; h. , 

 head; »»., "germinal vesicle," or 

 nucleus of the ovum; p.e., peri- 

 toneal epithelium. 



