220 Professor Owen's Teleology of the Skeleton of Fishes. 



are all persistent characters in osseous fishes, which remind the 

 anthropotoniist of transitional ones in the human foetus ; but the 

 light of teleology demonstrates the perfection of .such, so termed 

 embryonic 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 reference to the analogies to the form of a fish, we may be re- 

 minded that the head of the human embryo is disproportionately 

 large. True, 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 shew the radiating 

 osseous spiculse in their clear circumference, which is the active seat 

 of growth ; hence, the number of overlapping sutures, which least 

 oppose the progTessive extension of the bones. The cranial cavity 

 expands with the expansion of the head, the absorbents remove from 

 within as the arteries are extending the osseous walls without, but 

 the brain undergoes no corresponding 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 (xxiii. t. i., p. 309). 

 Now, this condition of the envelope 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 fa- 

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

 higher animal ever presents ; it is as peculiarly ichthyic, as it is ex- 

 pressly 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 supra-occipital, parietal, and frontal, above the cranium, and 

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

 nium in Lepidosiren, and the constant confluence of the posterior 

 and anterior basi-sphenoids in all bony fishes, disprove the constancy 

 of the supposed relationship, and lead us to look for other explana- 

 tions 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 vertebrae of 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 to swing freely to 

 and fro. The posterior and anterior sphenoids continue distinct 

 bones in all mammalia, during a period of life at which they form 

 one continuous bone in fishes. 



