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 
Fic. 30.—D1AGRAM OF ForMaTION OF aN Upricut Compounp RETINA. 
ABCD, as in Fig. 28. Op. g. I. and Op. g. II., two optic ganglia which combine 
to form the retinal ganglion, Rt. 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. 
