loO LECTURE VI. 



Hence the large cranial bones always show the radiating osseous spi- 

 culte in their clear circumference, 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 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 envelopes 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 facilitating its progressive motion, but it is 

 one which no embryo of a higher animal ever presents : it is as 

 peculiarly ichthyic, 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 supra-occipital, 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 posterior and 

 anterior basi-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 vertebrge 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 dui'ing a period of life at which they form one continuous 

 bone in Fishes. 



The flattened form of the frontal and parietal bones in Osseous 

 Fishes has been associated with the small develoi3ment of the brain 

 which they protect ; but observe how they would have impeded the 

 progress of the fish, had they been expanded into the dome-shaped 

 vault which arches over the skull of Birds and Mammals. There was 

 no need of that development in Fishes ; but we must not overlook the 

 fact that its very absence is a perfection in their structure, — an 

 adaptation to their sphere and mode of locomotion. 



The loose connections of most of the bones of the face may likewise 



