XIX] NERVOUS SYSTEM 417 



The essential element in the nervous system of Vertebrata, as in 

 all other nervous systems, is a kind of cell which has been variously 

 styled nerve-cell, ganglion-cell and neuron. This last name is 

 undoubtedly the best, as it avoids the old misapprehension that re- 

 garded the nerve-cell and nerve-fibre as two independent structures. 

 On page 62 it was pointed out that the nerve-fibre 

 s t?u h cti n o u f te is a very fine basal outgrowth of a modified ecto- 

 syltem! 0118 derin cell wliich is the nerve-cell. The cell, including 

 its outgrowth, is termed the neuron. Important 

 discoveries have recently been made on the minute structure of the 

 nervous system of Vertebrata, and we are now able to form a 

 simple and connected idea of the principles on which it is built up. 

 Originating as a simple strip of ectoderm wliich becomes rolled up 

 so as to form a tube, it is at first composed of cells which extend 

 through its entire thickness and which all abut on the cavity of 

 the tube. Some retain this position but develop branches and 

 deposit a large amount of cuticular substance in their cytoplasm : 

 these, constituting the supporting elements of the system, are termed 

 collectively neuroglia. Other cells retire from the cavity of the 

 tube, becoming more or less rounded in form, but developing a 

 number of outgrowths : these cells are the neurons. Each neuron 

 is provided with a number of branching processes, sometimes 

 arising from a single thick stem ; these are called receptive den- 

 drites (Gr. SeVSpoi', a tree), and they receive impulses. Impulses 

 are transmitted through one long basal process, called the axis- 

 cylinder process or axon, which ends in a tuft of processes 

 often thickened at the tips, which are called terminal dendrites. 

 The name axis-cylinder is suggested by the circumstance that 

 amongst Vertebrata this process is in many cases surrounded by 

 a fatty sheath of a conspicuous white colour, called the myelin; 

 a process with or without this sheath making up what is known 

 as a nerve-fibre. The tuft of dendrites in which the axon ends is 

 found to be in close contiguity either with the receptive dendrites of 

 another neuron, by which means the impulse is transmitted from one 

 neuron to another, or with a gland-cell which is thereby caused to 

 secrete, or else with the muscle-plate of a muscle-fibre, by which 

 means the fibre is stimulated. The muscle-plate is a disc of proto- 

 plasm with several nuclei situated at the side of the muscle-fibre. 

 The axon may give off several branches termed c ollaterals. These 

 like the main stem end in tufts of dendrites ; in this way an impulse 

 may spread over several paths. The receptive dendrites of a neuron 

 s. '& M. 27 



