596 CEPHALOPODA. 



arrangement for facilitating the movements of the eye. The serous 

 membrane which lines this cavity, after investing 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 eyeball, 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 even 

 passes over the anterior surface of the crystalline. This serous mem- 

 brane Cuvier, very improperly, named the "conjunctiva"; but, as Pro- 

 fessor Owen has suggested*, it is evidently rather analogous to the 

 membrane of the aqueous humour, here excessively developed in con- 

 sequence 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 intervention of 

 a minute orifice visible in the transparent tegumentary cornea. 



(1595.) 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 prolonga- 

 tions of the cranial cartilage, and are inserted into the sclerotic tunic. 



(1596.) It is always interesting to the physiologist to observe the 

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

 development 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 struc- 

 ture of the human ear are not a little curious. In the simplest aquatic 

 forms the central portion of the internal ear alone exists, imbedded in 

 the as yet cartilaginous cranium. Gradually, as in fishes, semicircular 

 canals, prolonged from the central part, increase the auditory surface, 

 but still have no communication with the exterior of the body. In rep- 

 tiles and birds, destined to perceive sonorous impressions in an aerial 

 medium, a tympanic cavity and drum are superadded ; and lastly, in 

 the Mammiferous orders, external appendages for collecting and con- 

 veying sound to the parts within, complete the most complex and per- 

 fect form of the acoustic instrument. 



(1597.) As far as is yet known, the Tetrabranchiate Cephalopods have 

 no distinct organ of hearing ; but in the Dibranchiata, an ear, lodged in 

 an internal cranium, for the first time presents itself to our notice, and 

 at the same time exhibits the lowest possible condition of a localized 

 apparatus adapted to receive sounds. 



(1598.) In the anterior and broadest part of the cartilaginous 

 cranium t, where its walls are thickest and most dense, are excavated 



* Cyclopaedia of Anatomy and Physiology, loc. cit. p. 552. 

 t Cuvier, M6moire sur la Poulpe, p. 41. 



