234 DR WALTER H. GASKELL. 



each ganglion being composed of nerve cells, nerve fibres, and 

 a fine reticulated substance called by Leydig in Arthropods 

 Punktsubstanz, and known in Vertebrates as Gerlach's network. 

 A further analysis resolves the whole system into a combination 

 of groups of neurons, the cells and fibres of which form the 

 cells and fibres of the ganglia, while their dendritic connections 

 with the terminations of other neurons, together with the 

 neuroo;lia cells, form the Punktsubstanz. As is natural to 

 expect, that part of the central nervous system which helps to 

 form the compound retina is built up in the same manner as 

 the rest of the central nervous system. 



Thus, according to Parker, the mass of nervous tissue which 

 occupies the central part of the optic stalk in Astacus is 

 composed of four distinct ganglia ; the retina is connected with 

 the first of these by means of the retinal fibres, and the optic 

 nerve extends proximally from the fourth to the brain. Each 

 ganglion consists of ganglion cells, nerve fibres, and Punktsub- 

 stanz, and, in addition, supporting cells of a neuroglial type. By 

 means of the methylene blue method and the Golgi method, it 

 is seen that the retinal end cells, with their visual rods, are 

 connected with the fibres of the optic nerve by means of a 

 system of neurons, the synapses of which take place in and 

 help to form the Punktsubstanz of the various ganglia. Thus, 

 an impulse in passing from the retina to the brain would 

 ordinarily travel over five neurons, beginning with one of the 

 first order and ending with one of the fifth. He makes five 

 neurons although there are only four ganglia, because he 

 reckons the retinal cell with its elongated fibre as a neuron of 

 the first order, such fibre terminating in dendritic processes 

 which form synapses in the Punktsubstanz of the first ganglion 

 with the neurons of the second order. 



Similarly the neurons of the second order terminate in the 

 Punktsubstanz of the second ganglion, and so on, until we reach 

 the neurons of the fifth order, which terminate on the one hand 

 in the Punktsubstanz of the fourth ganglion, and on the other 

 pass to the optic lobes of the brain by their long neuraxons — 

 the fibres of the optic nerve. 



He compares this arrangement with that of Branchipus, 

 Apus, Estheria, Daphnia, etc., and sliows that in the more 



