20 THE VERTEBRATE SKELETON. 



The cartilage bones of the occipital segments are derived 

 from the parachordals of the embryonic skull, those of the 

 parietal and frontal segments from the trabeculae. 



In front of the presphenoid the basicranial axis is con- 

 tinued by the mesethmoid. 



(2) THE SENSE CAPSULES. 



These enclose and protect the special sense organs. 



(a) Auditory capsule. 



The basisphenoid is always continuous with the basi- 

 occipital, but the alisphenoid is not continuous with the ex- 

 occipital as the periotic or auditory capsule is interposed 

 between them. Each periotic capsule has three principal 

 ossifications ; an anterior bone, the pro-otic, a posterior bone r 

 the opisthotic, and a superior bone, the epi-otic. 



These bones may severally unite, or instead of uniting 

 with one another they may unite with the neighbouring bones. 

 Thus the epi-otic often unites with the supra-occipital, and the 

 opisthotic with the exoccipital. 



Two other bones developed in the walls of the auditory 

 capsule are sometimes added, as in Teleosteans ; these are the 

 pterotic and sphenotic. 



(b) Optic capsule. 



The eye is frequently enclosed in a cartilaginous sclerotic 

 capsule, and in this a number of scale-like bones are often 

 developed. 



Several membrane bones are commonly formed around the 

 orbit or cavity for the eye. The most constant of these is 

 the lachrymal which lies in the anterior corner; frequently too, 

 as in Teleosteans, there is a supra-orbital lying in the upper 

 part of the orbit, or as in many Reptiles, a postorbital lying in 

 the posterior part of the orbit. 



(c) Nasal capsule. 



In relation to the nasal capsules various bones occur. 



