72 



THE ORIGIN OF VERTEBRATES 



At the present time, owing to the researches of Golgi, Ramon y 

 Cajal, and others, the nervous system is considered to be composed 

 of a number of separate nerve-units, called neurones, each neurone 

 consisting of a nerve-cell with its various processes; one of these 

 — the neuraxon— constitutes the nerve-fibre belonging to that nerve- 

 cell, the other processes— the dendrites — establish communication 

 with other neurones. The place where these processes come together 

 is called a synapse, and the tangle of fine fibres formed at a number 

 of synapses forms the ' neuropil.' 



When, therefore, a compound retina is formed by the amalgama- 

 tion of the ectodermal part— the retinal cells proper — with the 

 neurodermic part— to which the name 'retinal ganglion' may be 



Pig. 30. — Diagram of Formation of an Upright Compound Retina. 



ABCD, as in Fig. 28. Op. g. I. and Op. g. II., two optic ganglia which combine 



to form the retinal ganglion, Bt. g. 



given, — such a retina consists of neuropil substance and nerve-cells, 

 as well as the retinal end-cells. In all such compound retinas, the 

 retinal ganglion is not single, but two optic ganglia at least are 

 included in it, so that there are two sets of nerve-cells and two 

 synapses are always formed ; one between the retinal end-cells and 

 the neurones of the first optic ganglion, which may be called the 

 ganglion of the retina, the other between the first and second 

 ganglia, which, seeing that the neuraxons of its cells form the 

 optic nerve, may be called the ganglion of the optic nerve. The 

 ' neuropil ' formed by these synapses forms the molecular layers of the 

 compound retina, and the cells themselves form the nuclear layers. 

 Thus an upright compound retina, formed in the same way as the 

 upright simple retina, would be illustrated by Fig. 30. 



