GENERAL ORGANOLOGY. 



123 



rounded bimcli of regularly firriingod nerve-fibres and gangliou- 

 cells. The former constitute the centre of the mass, and, since 

 they cross in all directions, give the appearance of fine granula- 

 tions; this fact has led to the unsuitable, because misleading name 

 of 'Leydig's dotted substance.' The ganglion-cells, on the other 

 c 



Fig. 74.— Third abdominal ganfclion of a crayfish. (After Retzius.) C, coiiiiective or 

 longitudinal commissure; y, ganglion cell layer; f/' ganglion cell whose neurites 

 enter the connective; g'', «anglion cell whose neurites enter the peripheral nerve; 

 i, Leydig's dotted substance; N, peripheral nerve. 



hand, collect in a thick layer around tlie dotted substance. The 

 peripheral nerves, and also the commissure.'^, the cords connecting 

 similar ganglionic masses, extend outwards from the ganglia. 



SupraoEsophageal (or Cerebral) Ganglia. — Since most animals 

 are symmetrical, the ganglia occur in pairs; left and right ganglia 

 correspond to one another and are connected simply by a cord of 

 nerve-fibres, the transverse commissure. Of most constant occur- 

 rence are two ganglia, Avhicli lie dorsally al)ove the pharynx, and 

 hence are called the supracesophageal or cerebral fiajujUa. If other 

 ganglia occur, they lie ventrally and below the digestive tract 

 (ventral nerve-cord). 



