Prolocerebrum of Micropteryx. 117 



again concern us. They are formed by the fusion of four 

 segmental ganglia, the mandibular, the intercalary, the 

 maxillary and the labial. The intercalary ganglion has 

 hardly been noticed by insect neurologists ; the correspond- 

 ing appendage is the maxillula, which is vestigial or absent 

 in adult insects; the ganglion is accordingly ill-developed 

 or absent. The other three ganglia are mainly if not 

 entirely motor and sensory centres to their respective 

 appendages. 



All the nerve centres of insects consist of the following 

 layers.* They are bounded externally by a neurilemma, 

 which is a thin syncytial structure. Within this, and lying 

 loosely in a quantity of fluid, are the nerve cells, or ganglion 

 cells. The processes of these, the axons, pass inwards to 

 form the innermost part of the centre; here they unite 

 in very large numbers to form the tissue known as axonic 

 substance (or fibrillar material), which consists of innumer- 

 able axons and their twigs bound together by a varying 

 amount of neuroglia. Of this axonic substance two types 

 may be distinguished; the first is that which is called 

 Punktsubstanz, or Marksubstanz, or neurospongium ; its 

 composition was first accurately explained by Viallanes. 

 Until his day it had been known as a tangled web, but 

 in it he distinguished very fine axis cylinders ruiming 

 in all directions, and their twigs, and also the neuroglia. 

 In the second type of fibrillar substance, the Fasersub- 

 stanz, the axons run in bundles and form well-defined 

 tracts in which there is little or no neuroglia. The 

 distinction between these two types must not be insisted 

 upon, for every degree of intergradation may be found; 

 even in the most indisputable Punktsubstanz small tracts 

 of fibres may generally be detected. Physiologically again 

 the difference is one of degree, though Fasersubstanz 

 is mainly a tissue of conduction, Punktsubstanz one of 

 association, that is to say one in which impulses pass from 

 one neuron to another. 



Two parts of the brain may be connected either by 

 fusion of their component Punktsubstanz {Verschmehungen, 

 soudures), or by defbiite tracts of Fasersubstanz {Faser- 

 verhindungen). This distinction, again, has only a relative 

 value. 



Before we pass to examine the structure of the various 



* The general relation of neurilemma, cells and axonic substance 

 is shown on Plate X. 



