THE SKULL OF OSSEOUS FISHES. 85 



magnum ; the ajiex is more or less widely and deeply cleft trans- 

 versely by the aperture of the mouth ; the orbits arc lateral, largo, 

 and usually communicating freely with one another ; and there are 

 also two lateral fissures behind, called gill-apertures, Avith a mecha- 

 nism for opening and closing them. The mouth receives not only 

 the food, but also the streams of water for respiration, which escape 

 by the opercular or gill-apertures. The head contains not only the 

 brain and organs of sense, but likewise the heart and the whole re- 

 spiratory appai'atus. The inferior, inverted, htemal protecting arches 

 are greatly developed accordingly, and their diverging or radiated 

 aj^pendages support membranes re-acting upon the sui-rouuding fluid, 

 and more or less employed in locomotion : one pair, in fact, is the homo- 

 logue of the pectoral extremities in higher Vertebrata, and the sus- 

 taining (scapular) arch frequently also supports the homologues of 

 the pelvic extremities. Thus jaws and tongue, heart and gills, arms 

 and legs, may all belong to the head ; and the disproportionate size 

 of the skull, and its firm attachment to the trunk, required by these 

 functions, are precisely the conditions most favourable for facilitating 

 the movements of the fish through its native element. 



It may well be conceived, then, that more numerous bones enter 

 into the formation of the skeleton of the head of Fishes, than of any 

 other animals. Most of these bones present the squamous character 

 and mode of union, being flattened, thinned off, and overlapping one 

 another like scales ; and although the skull, as a whole, has less mo- 

 bility on the trunk than in higher animals, more of the component 

 bones enjoy independent movements. 



The principal cavities, which are formed by this assemblage of 

 bones, are, the ' cranium,' lodging the brain and organs of hearing ; 

 the ' orbital' and the ' nasal' fossae ; the ' buccal' and the ' branchial' 

 canals. Few of these cavities are well defined, and in no class of 

 animals is the exterior of the skull so broken by irregular depressions 

 and prominent spines and protuberances. The upper surface of the 

 cranium is commonly traversed by five longitudinal crests, inter- 

 cepting four channels : the principal crest is the median one, formed 

 by the frontal and occipital bones (Jiff. 19. 3); next to this is the 

 pair formed by the pai'ietals (ib. 7) and par-occipitals ; and the 

 lateral pair of crests is formed by the post-frontals and mastoids 

 (ib. 12, 8) : the intervening depressions lodge the anterior origins 

 of the great muscles of the back and of the scapular arch : very 

 rarely do the temporal muscles extend their attachments (as in the 

 Conger, Lepidosiren and Symbranchus) to the upper surface of the 

 cranium. The upper border of the oi'bit sometimes sends off" strong 



G 3 



