ANATOMY OF VERTEBRATES. 



97 



the neurapophysis, ' alisphenoid,' 6 ; the neural spine, e parietal,' 

 7 ; and the diapophysis, ' mastoid,' 8. The alisphenoids protect 

 the sides of the optic lobes, and the 

 rest of the penultimate segment of 

 the brain called ' mesencephalon ; ' 

 the mastoids project outward and 

 backward as strong transverse pro- 

 cesses, and give attachment to the 

 piers of the great inverted haemal 

 arch. Before noticing its struc- 

 ture, I may remark that, in the 



recent Cod-fish, the case, partly 



Disarticulated mesencephalic arch, viewed 

 from behind ; Cod (Morrhua vulgar ix) 



gristly, partly bony, which contains 

 the organ of hearing, is wedged 

 between the last and penultimate neural arches of the skull. 

 The extent to which the ear-case is ossified varies in different 

 fishes, but the bone is always developed in the outer-wall of the 

 case. In the Cod it is unusually large, and is called ' petrosal,' 

 fig. 81, 16; in the Perch, fig. 84, 16, and Carp, fig. 83, ig, it is 

 smaller : it forms no part of the segmented neuroskeleton. In the 

 acoustic organ which it contributes to enclose, there is a body as 

 hard as shell, like half a split almond : it is the ' otolite,' fig. 



81, 16. 



The hsemal arch consists of a pleurapophysis and a hamiapo- 

 physis on each side, and a hamial spine ; the pleurapophysis is in 

 two parts, the upper one called ' stylohyal,' ib. 38 ; the lower one 

 called e epihyal,' ib. 39 ; the hamiapophysis is called ' ceratohyal,' 

 ib. 40. The hamial spine is subdivided into four stumpy bones, 

 called collectively 'basihyal,' ib. 41 ; and which, in most fishes, 

 support a bone directed forward, entering the substance of the 

 tongue, called ' glossohyal,' ib. 42 ; and another bone directed 

 backward, called ' urohyal,' ib. 43. 



The ceratohyal part of the hamiapophysis supports an appendage, 

 or rudimental limb, called ' branchiostegal,' fig. 81, 44, answering 

 to the pectoral fin diverging from the haemal arch, in the adjoining 

 occipital segment. 



The penultimate segment of the skull above described is called 

 the ' parietal vertebra ; ' the neural arch is called s mesencephalic ; ' 

 and the hamial arch is called ' hyoidean ' in reference to its sup- 

 porting and subserving the movements of the tongue. 



The next segment, or the second of the skull, counting back- 

 ward, can be detached from the foremost segment without dividing 

 any bone. It is then seen to consist, like the third and fourth 



VOL. I. ir 



