NERVOUS CONSTITUENTS OF THE RETINA. 250 



is comparable to that of the rod fibres, and is in many animals 

 ] >recisely similar. In the rods and cones themselves we see the 

 terminal organs of the optic-nerve fibres. Whether the fibrils 

 in the interior of the inner segments stand in connection with 

 the nerve fibrils of the fibres in question, and form their modi- 

 fied extremities, must remain undecided, as well as the question 

 of the relations of the outer segments to the nerve substance. 

 It is highly probable that the internal and external segments 

 have a common sheath or investing membrane, but that they 

 are in any other way continuous, as, for example, by means of 

 nervous fibres in their interior, is at present a pure matter of 

 theory. Hence it is possible that the nervous substance may 

 terminate at or in the internal segment, and the outer segment 

 may represent a non -nervous physical accessory apparatus. 

 The relation presented in fig. 357 appears to be very note- 

 worthy and important for the explanation of the significance 

 of nerve cells in general; namely, that the processes of the 

 nervous cells of the retina always become thicker towards the 

 periphery than towards the centre. If this difference in 

 thickness depends on a difference in the number of the element- 

 ary nerve fibrils, the latter must become more numerous at the 

 periphery than at the centre, which can only be explained by 

 an augmentation of the number of fibrils within the nerve 

 cells. 



That the most minute peculiarities of structure presented by the 

 rods and cones of the retina claim our attentive consideration, is 

 obvious when we reflect that we have here to deal with structures that 

 are capable of converting the undulations on which light depends into 

 nerve force. 



We may and indeed must suppose that the structure of the terminal 

 organs is appropriate to their function, whilst the hope of discovering 

 something artificial in the rods and cones is rightly grounded and 

 supported on the fact that the more exact the research, and the sharper 

 the definition of the instruments employed, the more delicate and the 

 more remarkable are the details brought into view. No doubt but that 

 many persons, in consideration of the extreme shortness of the waves of 

 light, will regard this as too bold a statement. Yet if we bear in 

 mind that the length of these waves amounts to 0'7 of a micromillimeter 

 at the red end of the spectrum, and C'4 at the violet end, which ar v 



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