14S THE AMEBIC AN MONTHLY [July, 



stood by him, and we can read from our Figures 4 and 6 as is done by 

 the referencing in the plate. According to the results of his studies a 

 single nerve fibre passes up through the centre of the crystalline rod, 

 which is composed of fovn- bodies or rods placed together so as to sur- 

 round the axial nen'e. The outer end of the rod comes close against 

 the corneal facet, which, like the rod, is square across. The lower part 

 of the rod is composed of parts of varying density, and these give the 

 peculiar appearance to the pedicle. The rod is surrounded by two sets 

 of pigment bearing cells, one of which surrounds the base or pedicle, 

 and the other the crystalline cone. The former are called retinul<e. In 

 Figure 4 they appear to be very considerablv shrunken. According to 

 Fatten the pigment is collected in only part of a cell which runs up a 

 thin thread to the corneal hypodermis. In a row outside the eight retin- 

 ulae stand four pigment cell embracing the corneal hypodermis. These 

 are shown in Figure 6. These, according to Patten, are collocations of 

 pigment in the outer portion of very fine threads which run the entire 

 distance from the corneal hypodermis to the basilar membrane. In 

 Huxley's account (Vide Cray-fish, p. 119) the parts seem to compare 

 with Fig. I, as follows : 



Outer dark zone=pigment cells ; outer white zone=space between 

 pigment cells and retinulai ; middle dark zone = retinulae ; striated 

 spindles=pedicles ; crystalline cones^crystalline cones. 



While it is possible in a vague and general way to assign uses for 

 some of the parts of this very complex apparatus, it is not easy to be 

 specific. We may say that the corneal facets by their convexity act as 

 lenses and intensify the action of the rays of light by causing more of 

 them to act at once. The crystalline cone bv some means comes be- 

 tween the light stimuli and the nerve, which latter without it would not 

 be roused to activity by so weak a stimulation ; but how does the rod 

 do that.^ Perhaps the outer pigment cells prevent the passage of light 

 from cone to cone, an event which we can easily see would greatly in- 

 terfere with the localization necessary to distinct vision, but there is no 

 evidence that the cray-fish has distinct vision. Perhaps the inner pig- 

 ment cells are the seat of chemical changes which act as stimuli to the 

 crystalline rods. If so, why do the nerves run to the summits of the 

 cones as they seem to do from the most careful studies? It is not pos- 

 sible to positively say exactly what part each of these structures plays 

 in the cray-fish vision. 



We may, however, notice in general that this complicated system of 

 parts connected with the end of the sensory nerve is correlated with 

 other structures of various nature which have the common quality of 

 being apparatuses for the treatment of excitations as of light or sound, 

 which if left to opei^ate upon a bare nerve would be too feeble to atVect 

 it. Such are known as sensory end organs ; ears and touch organs, 

 and taste organs, as well as eyes, are constructed so as to utilize this 

 principle. 



Conclusion. — Since this paper will conclude the series on the cray- 

 fish, it is proper that before lea\ing the subject a few closing remarks 

 be made. The method of treatment in the series has been rambling 

 but not incoherent, for, as stated in the outset, the intention has been 

 to illustrate the method of histological studv rather than to make a 

 systematic course in histology. The comparative simplicity of some 



