204 VERTEBRATE ZOOLOGY 



Exercise 48. Draw the ventral aspect on a scale of 2, and carefully 

 label all these characters. 



The bones of the cranium, which have just been studied, fall 

 into two categories, those forming the cranium proper, or brain 

 case, and those forming the special-sense capsules. The follow- 

 ing bones belong to the cranium proper, or brain case : the occip- 

 itals, the sphenoids, and the ethmoid, which are cartilage bones ; 

 and the parietals, frontals, basitemporal, and rostrum, which are 

 membrane bones. The special-sense capsules are three in number. 

 The auditory capsule is formed by three cartilage bones, the 

 prootic, opisthotic, and epiotic, all of which in the adult bird 

 are fused with other bones. The prootic, in which the largest part 

 of the auditory capsule is lodged, is fused with the squamosal, a 

 large membrane bone that forms a part of the wall of the brain 

 case. The epiotic fuses with the supra-occipital, and the opisthotic 

 with the exoccipital. The optic capsule does not ossify, but the 

 lachrymal, a membrane bone, develops in connection with it and 

 forms a part of the anterior wall of the orbit. Besides these, a 

 row of small membrane bones called the sclerotic plates appear 

 in the sclera around the cornea. The nasal capsule is formed by 

 the mesethmoid, the nasals, and the vomer. 



The remaining bones of the skull, which form the upper and 

 lower jaws, the suspensorium, and the hyoid apparatus, belong 

 to the visceral skeleton. 



Study the hyoid apparatus. It is composed of a jointed median 

 body, and two pairs of slender projections called the horns or 

 cornua. The body is composed of three pieces. The anterior piece, 

 the basihyal, is the largest and is shaped like an arrowhead. 

 It is partly cartilaginous and partly bony, and from its hinder end 

 extends a pair of short projections, the anterior cornua. The 

 middle piece is of bone, and from its hinder end extends a pair 

 of long, slender projections, the posterior cornua, which are com- 

 posed principally of bone and are jointed. The posterior piece 

 is composed of bone and cartilage. 



Exercise 49. Draw the hyoid apparatus. 



