326 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



(ethmoid) of which is perforated for the passage of 

 the olfactory nerves from the brain. In the occipital 

 region, on either side, the capsule for the ear (periotic 

 capsule) early becomes connected with the cranial 

 walls, and in the higher Yertebrata (see Fig. 136) the 

 periotic cartilages are, at a very early stage in deve- 

 lopment, con- 

 tinuous with 

 the basilar 

 plate. The 

 optic cartilage 

 (sclerotic) 

 lies in the 

 sphenoidal re- 



CV 



gion, 



but it 



never enters 

 into direct 

 union with 

 the cranium, 

 though the 

 form of this 

 part of the 

 skull is 

 greatly affec- 

 ted by the 

 size of the eye- 

 ball. 



In addi- 

 tion to the (1) 

 cranial and 

 (2) sensory 

 cartilages 



which take so large a part in the formation of 

 the skull, there is in all Gnathostomata yet a third 

 element, which may be distinguished as the touccal. 

 In the branchial and visceral clefts, which appear 

 just behind the brain, cartilaginous bars are developed 



Fig. 136. Cartfagiuous Cranium of a Chick of 

 the fourth Day of Incubation, showing the 

 investing Mass (iv), and the Trabeculee (tr), 

 with their Central space (pts). 



cv, Cerebral vesicle (sliced off) ; e, eye ; Ig, anterior end 

 of investing mass formed from the parachordals ; 5, 

 notch for the passage of the fifth nerve ; q, quad- 

 rata ; cl, cochlea : she, semicircular canal of ear ; 

 9, foramen of exit of the ninth nerve ; we, noto- 

 chord. (After Parker.) 



