LOCOMOTION OF FISHES. 249 



logy demonstrates the perfection of such conditions, in relation to 

 the atmosphere and movements of the Fish. It is generally in 

 fresh-water abdominal Fishes that the semi-osseous condition of 

 the skull is found, and the diminution of the quantity of heavy 

 earthy particles may be connected with the less dense quality of 

 their medium, as compared with sea-water, and with the usually 

 more posterior position of the ventral fins. 



In the form of a fish, the head is disproportionately large, as it 

 is in the mammalian embryo. But the head of a fish must needs be 

 large to meet and overcome the resistance of the fluid, in the 

 mode most favourable for rapid progression : it must therefore 

 grow with the growth of the fish. Hence the large cranial bones 

 always show the radiating osseous spiculas in their clear circum- 

 ference, which is the active seat of growth ; hence the number of 

 overlapping squamous sutures, which least oppose the progressive 

 extension of the bones. The cranial cavity expands with the 

 expansion of the head : the absorbents remove from within as the 

 arteries build up from without ; but the brain undergoes no cor- 

 responding increase ; it lies at the bottom of its capacious chamber, 

 which is principally occupied by a loose cellular tissue, situated, 

 like the arachnoid, between the pia mater and the dura mater, and 

 having its cells filled with an oily fluid, or sometimes, as in the 

 Sturgeon, by a compact fat, 1 Now, this condition of the en- 

 velopes of the brain is not only, like the fibrous tissue and 

 squamous sutures of the ever-growing cranial bones, related to 

 the requisite proportions of the fore-part of the fish for facilita- 

 ting its progressive motion, but it is one which no embryo of a 

 higher animal ever presents : it is as peculiarly piscine, as it is 

 expressly adapted to the exigencies of the fish. 



It has been held that confluence of distinct bones is a consequence 

 of high circulating and respiratory energies ; yet the anchyloses 

 of the superoccipital, parietal, and frontal above the cranium, and 

 of the basi-occipital, basi-sphenoid, and pre-sphenoid below the 

 cranium, in Lepidosiren, and the constant confluence of the basi- 

 and pre-sphenoids in all bony fishes, disprove the constancy of the 

 supposed relationship, and lead us to look for other explanations 

 of such coalescence of primitively or essentially distinct bones. 

 We shall find a final cause for the rapid consolidation and union 

 of the elongated bodies of the two middle cranial vertebra? of 



o 



Fishes in the necessity for strength in the basis of that part of 

 the skull, from the sides of which the large and heavy mandibular 

 and hyoid arches and their appendages are to be suspended, and 



1 XXIIL t. i. p. 309. 



