THE NERVOUS SYSTEM 113 



receptor or sense organ to the ganglion, through an association 

 neurone to a motor neurone, and thence to a muscle or other effector 

 organ. Stimulation of the sense organ will thus produce contraction 

 in the muscle. This path of nervous conduction is termed a 'reflex 

 arc' and the response a simple reflex. The reflex arc is a physiological 

 abstraction; for even in the simplest response the course of conduc- 

 tion must be infinitely more complex, involving inhibition of opposing 

 muscles and compensatory movements elsewhere in the body. More- 

 over, the course of reflex conduction is not fixed; as the synapses 

 become 'fatigued' or 'adapted' the transmission may be blocked or 

 follow some other reflex arc with a lower threshold ; and if the stimuli 

 are excessively strong they may overflow into many paths and pro- 

 duce a discharge of impulses from a large group of motor neurones. 

 Throughout the nervous system the neurones, both cell bodies and 

 axons, are invested and insulated from one another by cellular 

 sheaths formed by the glial cells (Schwann cells). Only at the synapses 

 do the nerve fibres come into intimate contact with one another. In the 

 central ganglia, modified glial cells named 'perineurium cells' surround 

 the entire nervous system and secrete over it a tough connective tissue 

 sheath, the 'neural lamella'. Other glial cells invest the cell bodies, 

 which occupy the peripheral parts of the ganglia, and the interlacing 

 nerve tracts which form the central core, or 'neuropile', where the syn- 

 apses occur. There is no circulation of haemolymph within the 

 ganglia : the supply of salts and nutrients and the removal of waste 

 products are effected solely by translocation through the glial sheaths. 



The nerve cord 



The central nervous system of insects generally shows a well-marked 

 segmentation. Typically, each somite has its own pair of nerve centres 

 or ganglia, giving off sensory and motor nerves and connected to the 

 adjacent ganglia by a paired cord containing only nerve-fibres. In 

 virtue of these ganglia, each body segment enjoys a considerable 

 degree of autonomy; probably each has its own respiratory centre 

 controlling the movement of the corresponding spiracles, and each 

 is capable of carrying out such reflex movements as do not involve 

 the activity of other segments. But there is always some degree of 

 fusion between adjacent ganglia; in the higher forms the abdominal 



