no ANATOMY OF VERTEBRATES. 



Scomberoitls, and is very remarkable in most fishes with lofty 

 comx^ressed skulls, as the EjMpptis. In them it resembles, but is 

 not homologous with, the posterior prolongation of the nasal pas- 

 sages in the Crocodiles, and it lodges some of the muscles of the 

 ej'xjball. The form of the alisphenoids is influenced by that of the 

 skull ; when this is low and flat, their antero-posterior exceeds their 

 vertical extent ; in deep and compressed skulls they are narrow and 

 high plates ; in ordinary shaped skulls they present either a sub- 

 circular form, and are perforated, as in the Carp, fig. 8.3, 6, or arc 

 reniform, the anterior border being deeply notched, as in the Cod, 

 fin-. 81, c; they form a more definite and fixed proportion of the 

 lateral parietes of the skull than do the petrosals, ib. 16, which 

 arc interposed between them and the exoccipitals ; and they have 

 their essential function in sustaining and protecting the sides of 

 the mesencephalon, and in affording exit to the second and third 

 divisions of the fifth pair of nerves. The alispheuoid articulates 

 in the Cod with the petrosal posteriorly, with the orbitosphenoid 

 anteriorly, and with the mastoid and postfrontal alcove. ^Fhere 

 the alisphenoids have a greater relative size, as in the Perch, and 

 where the less constant jietrosal decreases or disappears, their 

 connections arc more extensive ; they then reach the exoccipitals, 

 and sometimes even join a small part of the basioccijiital. In 

 the incompletely ossified skulls of some fishes, e. g. the Pike and 

 the Salmon tribe, the basal and lateral cranial bones are lined by 

 cartilage, which forms the medium of union between them, 

 especially the lateral ones : in better ossified fishes, e. g. the Cod, 

 the union of the alisphenoids is by suture, partly dentated, partly 

 squamous. In the Cod the second and third di^-isions of the tri- 

 geminal nerve pass out of the cranium by the anterior notch ; in 

 some other fishes they escape l_)y foramina in the alisphenoid : a 

 part of the vestibule and the anterior semicircular canal of the 

 acoustic labyrinth usually encroach upon its inner conca^ity, 

 whence some have deemed it to be the petrous bone. The chief 

 variety in the parietals, figs. 76 and 83 7, has been noted in con- 

 nection with the superoccipital, \h. 3. 



In some fishes the parietal is perforated by the ' ncrvus 

 lateralis,' which supplies tlie vertical fins. The left parietal is 

 broader than the right in the Halibut and some other flat fishes 

 (^Plctironecfidce^. 



Tlie process fiir tlie attachment of the great trunk-muscles is 

 developed from the outer uiargin of the mastoid, figs. 83, 8,5, s ; 

 the inner side of this bone is expanded, and enters'^ slightly into 

 the formation of the walls of the cranial, or rather of tlie acoustic 



