92 ANATOMY OF VERTEBRATES. 



regarded as a mesially divided neural spine (nasal). Thus may be 

 discerned four cranial segments having the essential characters and 

 relations of the neural arch of the tyjoe vertebra. The upper, 22, 

 and lower, 30, jaws, the hyoid, 40, and sca2oulocoracoid, 50-52, fig. 42, 

 constitute four inverted arches ; but their vertebral relations will 

 be better understood in the composition of the skull in bony fishes. 

 § 30. Skull of Osseous Fishes. — The head is larger in proportion 

 to the trunk in fishes than in other vertebrate classes ; it is usually 

 in forru of a cone, figs. 34, 38, whose base is vertical, directed back- 

 ward, and joined at once to the trunk, and whose sides are three in 

 number, one sujierior, and two lateral and inferior. The cone is 

 shorter or longer, more or less compressed or squeezed from side 

 to side, more or less depressed or flattened from above downward, 

 with a sharper or blunter aj)ex, in different species. The l^ase of 

 the skull is perforated by the hole, called ' foramen magnum,' for 

 the exit of the spinal marrow ; the apex is more or less widely and 

 deeply cleft transversely by the ajierture of the mouth ; the eye- 

 sockets or 'orbits,' ib. 17, are lateral, large, and usually with a 

 free and wide intercommvmication in the skeleton ; the two 

 vertical fissures behind are called ' gill-slits,' or branchial or oj^er- 

 cular apertures ; and there is a mechanism like a door, ib., 35, 36, 

 37, for opening and closing them. The mouth receives not only 

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

 escape by the gill-slits. The head contains not only the brain and 

 organs of sense, but likewise the heart and breathing oro-ans. 

 The inferior or ' ha3mal ' arches are greatly developed accordingly, 

 and their diverging appendages support membranes that can act 

 upon the surrounding fluid, and are more or less emjjloyed in 

 locomotion : one pair of these appendages, ib. P, 50, 56, answers, 

 in fact, to the fore-limbs in higher animals ; and their sustainino- 

 arch, ib. 51, 52, in many fishes, also supports the homologues of 

 the liind-limbs, v, 70. Thus brain and sense-organs, 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 course of the fish 

 through its native element. 



It may well be conceived, then, that more bones enter into the 

 formation of the skull in fishes than in any other animals ; and the 

 composition of this skull has been rightly deemed the most 

 diflicult problem in Comparative Anatomy. ' It is truly remark- 

 able,' writes the gifted Okcn, to whom we owe the first clue to its 

 solution, ' what it costs to solve any one problem in Philosopliieal 



