ORGAN OF SIGHT IN MAMMALIA. 20 J 



C. Parallel between eye and ear. — The author of the excellent 

 articles, xcvn" and ex" has drawn a parallel between the eye and 

 car which, in the main, appears to me to express justly the 'serial 

 homologies ' of the parts of those sense-organs. I include, how- 

 ever, the consideration of the cavities in which they are respec- 

 tively lodged. The ' otocrane ' parallels the ' orbit.' The homo- 

 logy is masked by the deeper situation of the former, its commu- 

 nication rather with the interior than with the exterior of the 

 cranium, and its more frequent coalescence with the fixed bony 

 sense -capsule which it includes. In some Mammals, however, 

 that capsule retains its primitive and typical distinctness, and 

 can be removed from the otocrane. 1 This is, then, seen to 

 be formed by the exoccipitai and alisphenoid, the mastoid, the 

 tympanic, and, in Mammals, the expanded and intercalated squa- 

 mosal. The primitive bony nuclei of the capsule which appear 

 round the fenestra rotunda, on the outer end of the upper vertical 

 semicircular canal, and on the middle of the hinder vertical 

 semicircular canal, extend to form the bony labyrinth, and are 

 wholly independent of the centres from which the ossification of 

 the mastoid or of.ier otocranial bones begins. The addition of 

 bony matter envelopes in various degrees the first formed part of 

 the capsule, called i bony labyrinth,' and constitutes, therewith, the 

 6 petrosal.' This capsule of the ear corresponds with the sclerotic 

 in the eye ; which, in many Vertebrates, becomes the seat of 

 ossification, and in some (Cetacea, e.g., fig. 195, a) is thickened as 

 much out of proportion to the nervous and vascular parts of the 

 essential organ it contains, as is the petrosal. The orifice by which 

 the optic nerve enters the eye-bulb answers to the foramen audi- 

 torium internum. The membranous labyrinth answers to the 

 parts of the eyeball within the sclerotic. The delicate vascular 

 external tissue of the labyrinth, frequently exhibiting pigment- 

 specks, answers to the choroid, the expansions of the acoustic 

 nerves to the retina, the endolymph to the vitreous humour. The 

 fluid in the space between the sclerotic and choroid, including the 

 aqueous humour, represents the perilymph. Wharton Jones 

 compares the ' lens ' to the ( otolites.' 2 



If we compare the conjunctival space in front of the eyeball with 

 the tympanic cavity, and the duct therefrom leading to the nose 

 with the eustachian tube, then the anterior opening of the sclerotic 

 will answer to the fenestra vestibuli, and the membrane closing 

 it, or cornea, to that which closes the fenestra. In mammals 

 the open movable eyelids seem very remote analogues to the 



1 xliv. p. 557. - xcvn". p. 562. 



