470 CEPHALOPODA. 



numerous nervous filaments, which, after perforating the poste- 

 rior part of the globe of the eye, expand into the post-pigmental 

 retina. 



Between the globe of the eye (g) and the cornea (/) is a 

 capacious serous cavity, which extends to a considerable distance 

 towards the posterior part of the orbital chamber ; and holds the 

 same relation to the visual apparatus, and the cavity in which it is 

 lodged, as the serous lining of the human pericardium does to the 

 heart, and the fibrous capsule in which that viscus is lodged, evi- 

 dently forming an arrangement for facilitating the movements of 

 the eye. The serous membrane which lines this cavity, after in- 

 vesting the inner surface of the cornea and the interior of the orbit, 

 is reflected upon the outer surface of the sclerotic tunic of the eye, 

 which it likewise covers ; and moreover, at the front of the eye- 

 ball, enters the aperture which in the eye of a vertebrate animal 

 would be occupied by the cornea, lines the chamber corresponding 

 with that of the aqueous humour, and passes over even the anterior 

 surface of the crystalline. This serous membrane Cuvier very 

 improperly named the " conjunctiva ;" but, as Professor Owen has 

 suggested,* it is evidently rather analogous to the membrane of 

 the aqueous humour, here excessively developed in consequence of 

 the want of a cornea in the sclerotic aperture. This serous cavity is 

 not, however, a completely closed sac ; but, as is frequently the 

 case with the serous membranes of fishes and reptiles, is in com- 

 munication with the surrounding medium, through the interven- 

 tion of a minute orifice visible in the transparent tegumentary 

 cornea. 



(511.) Four muscular slips are appropriated for the movements 

 of this remarkable eye, and serve to direct the axis of the organ 

 so as to ensure distinct vision : they arise principally from the 

 orbital prolongations of the cranial cartilage, and are inserted into 

 the sclerotic tunic. 



(512.) It is always interesting to the physiologist to observe 

 the earliest appearance of a new system of organs, and witness the 

 gradual developement of additional parts, becoming more and 

 more complicated as we advance from humbler to more elevated 

 grades of the animal creation. The progressive steps by which 

 the auditory apparatus of the Vertebrata attains to that elaborate 

 organization met with in the structure of the human ear are not a 

 little curious. In the simplest aquatic fonns the central portion 



* Cyclop, of Anat. and Phys. loc. cit. p. 552. 



