Chap. VIII.] THE SKELETON. 151 



Of the anterior segment of the brain-case the frontal bones 

 are the most conspicuous and the earliest formed (fr.). They 

 constitute the whole of the central arch of the skull, and give 

 rise to a prominent supra-orbital ridge, of which, however, the 

 postero-inferior portion is formed by the ali-sphenoid. From 

 the ridge each is continued sharply inward, within the orbits, 

 as the orbital plate of the frontal. These orbital plates, and 

 the ali-sphenoids which lie below them, are in the chick separated 

 from the inter-orbital septum by a membranous tract, below 

 which is the optic foramen (op.). In this tract the orbito- 

 sphenoids make their appearance between the second and third 

 months of chick life. Anterior to them is the 9artilaginous 

 inter-orbital septum which contains a large membranous inter- 

 orbital fenestra. Behind this fenestra between it and the 

 optic foramen the pre-sphenoid (ps., the reference line passes 

 through the small orbito-sphenoid) begins to ossify at about the 

 ninth month, thus completing the frontal segment. In front 

 of the fenestra is the mesethmoid ossification. Beneath the 

 mesethmoid and the inter-orbital septum, stretching back along 

 the mid-line of the base of the skull as far as the basi-temporal, 

 is a grooved splint of bone, the rostrum (r.). Anterior to this is 

 the grooved vomer (vo.). 



In the adult fowl all these various skull-elements, except the 

 vomer, run together, and fuse into a continuous bony mass in 

 which the sutures are completely obliterated. The bones of the 

 three segments, the basi-temporal and rostrum below, the 

 squamosal and bones of the auditory capsules at the sides, the 

 mesethmoid in front, all unite firmly together in the skull of 

 the adult fowl, The inter-orbital septum becomes a continuous 

 plate of bone, except where the olfactory and optic foramina 

 place the two orbits in communication. And the f rentals 

 fusing with the nasals anteriorly, the cranial and facial portions 

 of the skull become continuous. 



The foramina for the exit of the nerves have the same 

 essential relations as in the rabbit. The student should com- 

 pare the two. 



There is no olfactory fossa. The cerebral fossa is large, and 



