ANATOMY OF VERTEBRATES. 115 



performance in the lower than In the higher classes, we find in 

 Fishes several bones taking part with the special acoustic capsule 

 in the lodgment of the labyrinth ; and it is only in the higher 

 Vertebrates that the capsule, under the name of the ' i)etrous 

 bone,' entirely and exclusively cnveloj^es the labyrinth. Its 

 ossification commences later than that of the cranial neurapo- 

 ])hyses, in the series of Osseous Fishes : there are sjoecies (c. g. 

 Pike) in which, after the exoccipitals, alisphenoids, and orljito- 

 sphenolds have received their destined amount of ossification, the 

 petrosal still remains in the cartilaginous state : it is small in the 

 Carp, fig. 83, 16, and Bream; in the Perch, figs. 84, 85, ic, it is 

 more developed ; it is somewhat larger in the flat-fish (e. g. 

 Halibut); and in the Cod, fig. 81, ic, attains an equal size with 

 the alisphenoid, ib. g, which it resembles in form, except that the 

 notched margin is posterior. Here it forms the posterior lateral 

 wall of the cranium ; articulates below with the basioccipital i, and 

 basisphenoid, above witli the mastoid 8, and paroccipital 4, behind 

 with the exoccipital 2, and before with the alisphenoid 6 : it sup- 

 ]x>rts the cochlear division of the labyrinth containing the ot(_>lites. 

 The cavity called ' otocrane ' lodging the jietrosal with the rest 

 of the ear-capsule, is formed, on each side, by the exoccipital, 

 paroccipital i, alisphenoid 6, mastoid 8, and postfrontal 4 : it is some- 

 times closed externally, but oj^ens widely into the cranial ca^•ity. 



The optic capsule, or sclerotal, fig. 81, 17, like tlie acoustic caj)- 

 sule, is cartilaginous in Plagiostomes, and also in the semi-osseous 

 fishes, as in most Granoids, the Lepidosiren, the Lophius, the 

 Lo] )liobranchs and Plectognathes. In better ossified fishes it is 

 bony, and commonly consists of two hollow hemispheroid pieces, 

 each with two opposite emarginations ; the inner ones circum- 

 scribing the hole, (analogous to the meatus internus of the 

 petrosal), for the entry of the nerves and vessels to the essential 

 parts of the organ of vision ; and the outer or anterior emargina- 

 tions supporting the cornea. As this part of the skeleton of the 

 head retains its primitive fibro-membranous condition in J\Ian, it 

 is called ' the sclerotic coat of the eye ; ' and the osseous plates 

 developed in it in Birds, many Reptiles, and Fishes, are termed 

 ' sclerotic bones.' It bears, however, the same essential relation 

 to the vascular and nervous parts of the organ of sight, which the 

 petrous bone does to those parts of the organ of hearing, and 

 whicli the turbinal bones do to the organ of smell : the })er- 

 sistent independence of the eye-capsule, which has led to its being 

 commonly overlooked as part of the skeleton, relates to tlic 

 requisite mobility and free suspension of the organ of vision. In 



