1870.] Organs of Vision in the Common Mole. 325 



by the hair so completely that it is difficult sometimes to discover it. In 

 removing the skin the small globe is easily detached at the same time, and 

 no indication remains of the exact position in which it was situated. This 

 shows that in the Mole the cavity of the orbit is wanting, and that the 

 structures usually found in the vicinity of the eye are in a different condition 

 from that which they present in other mammalia. It is necessary, there- 

 fore, to divide the skin around the base of the eye in order to preserve the 

 connexion between the globe and the subjacent tissues. 



Beneath the eye, and forming a basis on which it rests, is tc a firm mass 

 of cellular fibrous tissue which assumes on dissection a fusiform shape, with 

 an attenuated portion passing towards the base of the skull." The filament 

 becomes so exceedingly delicate in the deeper part of the orbit that the 

 difficulty of ascertaining its precise condition is probably the reason of the 

 difference of opinion on the subject. 



In Mr. Solly's specimen there was found to be no attachment whatever 

 of the filament to the base of the skull ; but in a former dissection of a 

 smaller, and probably younger specimen, the continuity between the bone 

 and the tissue was evident. 



The filament of tissue above described, and the connexion which it 

 formed between the eye and the skull, induced me to examine it microsco- 

 picallv, in order to ascertain whether it contained nervous fibres, or 

 possessed any of the characters of the optic nerve. 



It exhibited a tendency to divide in a longitudinal direction when needles 

 were applied to it, and presented the appearance of cellular tissue, without, 

 however, any trace of nerve-fibre. It will be seen nevertheless, from the 

 description of the optic nerve in the fcetal Mole, that this delicate thread 

 is the only vestige which remains of that important part of the organs of 

 vision in the full-grown Mole. 



With regard to some minute branches of nerves and blood-vessels which 

 pass into the tissue forming the base of the eye, both on its outer and 

 inner side, it is not in my power to say definitely from whence they come, 

 as their minute size prevented me from tracing them in the deeper part of 

 the orbit to their points of exit from the skull. 



The eye of the full-grown Mole presents a surface uniformly black and 

 glistening, in which there is no indication of a cornea and sclerotic distinct 

 from one another, nor any evidence of an iris or pupillary aperture. 

 Within the globe, when ruptured with the points of needles, a layer of 

 black pigmentary particles was found to line the internal surface of the 

 dense structure which corresponds to the sclerotic. 



In addition there was a confused mixture of grey and white granular 

 substance, in which there was no distinct evidence of remains of the usual 

 contents of the globe of the eye, though, as will be seen, those structures 

 exist in foetal life. 



The specimens were sent to me preserved in alcohol, consequently the 

 brain was firm, and easy to be removed entire from the cranium. 



