THE FROG 



87 



-N.R. 



-A.C. 



The other processes coming from the cell are termed the dendrons, 

 and as a rule are shorter and branch more freely, forming a tree-like 

 structure. They are the receptive processes and convey the stimuli 

 to the cells. The processes of various related ganglion cells come 

 very close to one another in most cases, and in 

 some appear to come into actual contact, so 

 that the nerve impulses, as the nerve messages 

 are called, can be transferred from the axon 

 of one cell to the dendrons of other cells 

 adjacent to it. The whole structure, cell body, 

 dendrons and axon, is termed a neuron, and 

 the nervous system is composed of countless 

 numbers of such closely bound up together, 

 with other cells of a supporting nature, the 

 neuroglia cells. Three main types of neurons 

 are met with and are distinguished by the 

 number of processes given off. If but one 

 process is present, the cell is termed unipolar. 

 Such cells are found, but not plentifully, in 

 the dorsal root ganglia, and as the single 

 process divides into two branches, an axon 

 and a dendron, they may perhaps be regarded 

 as a specialised form of the second group. 

 The next group are termed bipolar, as they 

 have but one dendron in addition to the axon, 

 and these too occur in the dorsal root ganglia. 

 All other cells with more than two processes 

 are termed multipolar, and are found through- 

 out the whole of the ventral nervous system 

 and are particularly well marked in the ven- 

 tral horn of grey matter in the spinal cord. 

 The actual form of the neuron varies in dif- 

 ferent parts, and it is possible to tell within 

 certain limits from what part of the brain a 

 section is taken, from the shape and arrange- 

 ment of its ganglion cells. 



The axons and dendrons, especially 

 if they have to travel far from the parent 

 cell, are covered with a thin but moderately 

 tough transparent membrane variously known as the neurilemma, 

 the primitive sheath or sheath of Schwann. The nuclei belonging 

 to this covering are scattered irregularly on its inner surface between 

 it and the nerve fibre, which is termed the axis cylinder. The whole 

 structure is of a greyish colour and is known as a grey fibre or, in 



-N. 



-M.S. 



-c.s. 



-A.C. 



FIG. 29. Portions of 

 two nerve fibres. 



A.C., axis cylinder ; 

 C.S., cleft of Schmidt; 

 M.S., medullary sheath ; 

 N., nucleus ; N.R., node 

 of Ranvier ; P.S., primi- 

 tive sheath. 



