IV. VERTEBRATA 



459 



basal bones, (fig. 519), basioccipital, basisphenoid, presphenoid, and 

 mesethmoid ; right and left of this a row of exoccipital, alisphenoid, 

 orbitosphenoid, and ectethmoid. The position of the otic capsule results 

 in the sum of the otic bones, the petrosal, being wedged in between 

 the exoccipitals and the alisphenoid. Only behind is there a dorsal 

 element, the supraoccipital. 



The skull is roofed in Ijy membrane bones, and of these three pairs are 

 almost constantiy present. These are, from behind forwards, a pair of 

 parietals, a pair of Jrontals, and a pair of nasals, the latter covering the 



Fig. 518. Fig. 519. 



Fig. 518. — Sagittal section of hinder part of goat skull (from GegenbaurJ. For 

 lettering see fig. 517. 



Fig. 51Q. — Diagram of floor of skull of infant. /, mesethmoid; i, ectethmoid; //, 

 presphenoid; 2, orbitosphenoid; ///, basisphenoid; 3, alisphenoid; IV, basioccipital; 

 4, exoccipital. Other letters as in fig. 517. 



nasal capsules. The lower vertebrates have a large membrane bone on 

 the floor of the skull, the parasplienoid, which reaches from the basioccip- 

 ital to the mesethmoid. 



The scheme of the cranium thus outlined undergoes the most modifications 

 in the sphenoidal region. Parasphenoid, on the one hand, and basi- and 

 presphenoid, on the other, may be substituted for one another, so that when the 

 parasphenoid is present (fishes, Amphibia) the others are small or absent and 

 vice versa (mammals). In the mammals, besides, the alisphenoids fuse with 

 the basisphenoid (greater wings), the orbitosphenoids with the presphenoid 

 (lesser wings), so there arise here an anterior and a posterior sphenoid, fused in 

 man and some other mammals to a sintjle sphenoid bone. Mesethmoid and 

 ectethmoids likewise fuse in the mammals to an ethmoid bone. The existence 

 of these four complexes was the basis of the view (p. 455) that the skull is 

 composed of vertebrae. 



The brain case, or cranium, is developed into the complete skull by the 

 addition of the visceral skeleton, a series of arches which embrace the begin- 



