1870.] 



Organs of Vision in the Common Mole. 



3.23 



inserted into the posterior part of the globe of the eye in the line of the axis 

 of vision." 



In 1813 Tiedemann published a description of the optic nerve and 

 the fifth pair, which differed in a very important respect from the account 

 given by Zinn ; for he says that although the optic nerves are small and 

 difficult to distinguish, yet they exist as separate nerves, and present the 

 same general character as in most of the mammalia. Tiedemann carried 

 his investigations still further, and declared the absence of the third, fourth, 

 and sixth pairs of nerves. He described certain filaments, which he stated 

 to be unconnected with the optic nerve, and to be similar to those branches 

 which are found in the tissues around the eye in other animals. The ab- 

 sence of the third, fourth, and sixth pairs of nerves was subsequently 

 asserted also by Cams ; but his account of the origin and termination of the 

 optic nerves is not quite intelligible to me. It appears, however, that the 

 conclusions of Treviranus and Carus agreed that there was some connexion 

 between the optic nerve and the fifth, which sufficed to supply the Mole 

 to a certain degree with the sense of sight. Indeed the chief physiolo- 

 gical fact which Treviranus endeavoured to establish in the chapter of the 

 work alluded to, was that the nerves of one particular and special sense 

 were capable, under certain circumstances, of becoming endowed with the 

 properties of nerves of another and different sense. "The fifth pair of 

 nerves in some mammalia supply the place of the most important nerve of 

 sense " is the introductory sense in the chapter ; or, in other words, that a 

 nerve of touch and feeling may become a nerve of sight, that is, sensitive 

 to the rays of light ; and he concludes the chapter thus : — " I cannot but 

 agree with Carus that the optic nerve and the fifth branch enter into con- 

 nexion in the eye to produce the retina." This opinion met with opposi- 

 tion from Prof. Miiller, who controverted it by the statement that true 

 optic nerves had been exhibited to him by Dr. Henle (Baly's translation, 

 p. 842). 



From a remark of M. P. G. Pelletan, in his ' Memoire sur la Speciality 

 des Nerves des Sens,' quoted by Mr. Solly in his work on the Brain, it 

 would appear that that anatomist had made a very careful examination of 

 the organs of vision, both in the adult and foetal Mole, for he " recom- 

 mends the dissection of either foetal Moles, or very young ones, in whom 

 the optic foramen is still distinct." The importance of this remark con- 

 sists in the proof that Pelletan had observed that the optic foramina 

 undergo some change subsequent to the birth of the animal. 



Von Sieboldhas published the results of investigations into the difference 

 between the eyes of certain species of Talpa. " The eyes are rudimentary," 

 he says, "in the Mole and Spalax typhlus, which live underground; and 

 above all in Talpa cceca and the Chrysochlores are the eyes rudimentary. 

 They are a little more developed in the Musaraignes and the Common 

 Mole. According to Ollivicr (Bulletin de la Societe Philomathique, vol. ii. 



