The Retina and Optic Ganglia in Decapods, especially in Astaciis. 37 
5. Optic Ganglia. 
a. Composition. 
The mass of nervous tissue that occupies the centrai part of 
the optic stalk is composed of four distmct ganglia so placed that 
they constitute a series extending from the retina toward the brain 
(PI. 1 Fig. 27). These ganglia have been designated, beginnìng with 
the most distai one, as the first, the second, the third, and the fourth 
optic ganglion (I— IV). The retina is connected with the first of 
these by the retinal fibres (fbì\r), and from the fourth the optic 
nerve {ti.opt) extends proximally to the brain. The three Spaces 
between the ganglia are bridged over by Systems of nerve fibres. 
The chief constituents of the ganglia are ganglionic cells, nerve 
fibres, and »Punktsubstanz«; but, in addition to these, the ganglia 
also contain capillaries, connective tissue cells, and possibly a neu- 
roglia. 
The capillaries that ramify in the substance of the ganglia 
are derived from the subdivision of a single small artery that makes 
its way over the posterior face of the ganglionic mass. In pre- 
parations in which these capillaries have been injected with haema- 
toxylin, it is easy to demonstrate the distribution of these vessels. 
They are about as abundant in the fibrous regions between the 
ganglionic centres as in these centres themselves, and within 
each ganglion they are noticeably more numerous in the «punkt- 
substanz« than among the surrounding ganglionic cells. 
Connective tissue cells occur not infrequently throughout 
the ganglia, being rather more abundant on the periphery of these 
structures and among the nerve fibres between them than in other places. 
The presence of a neuroglia like that in Vertebrates will be con- 
sidered in connection with the first ganglion, in which this kind of 
tissue probably occurs. 
The ganglionic cells are superficial in position, forming an 
incomplete external coating to each ganglion. They are always 
absent from the distai and proximal faces of these structures, and, in the 
cases of the second and third ganglia, from the posterior faces also 
(PI. 1 Figs. 34, 35, 36); the remainder of the ganglionic surfaces is 
covered with cells. The ganglionic cells can be satisfactoiily studied 
in preparations stained with methylen blue, and in such preparations 
two types of cells can be distinguished : apolar and unipolar. 
The apolar cells, which are unquestionably shown to be such 
