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 

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

 the skull is found, and the diminution of the (|uantity of heavy 

 earthy particles may be connected with the less dense quality of 

 their medimn, 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 disproportionate^ 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 favoural:)le for rapid progression : it must therefore 

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

 always show the radiating osseous s})icidiB in their clear circum- 

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

 overlapping squamous sutiu-es, which least ojipose 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 u]) 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, Ijy a com})act fat.' Now, tins condition of the en- 

 veloi)cs of the brain is not only, like the filirous tissue and 

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

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

 ting its progressive motion, luit 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 superocclpital, parietal, and frontal above the craniiuu, and 

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

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

 and pre-sphcnoids 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 Ijoues. 

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

 of the elongated bodies of the two middle cranial -s'ertebra; of 

 Fishes in tlie 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 



' xxiu t. i. p. 309. 



