ii4 MORPHOLOGICAL TYPES 



vibrations brought by the columella auris are imparted through 

 the fenestra ovalis to the perilymph, which in turn passes 

 them on through the wall of the auditory vesicle to the 

 endolymph. Here the vibrations stimulate the special sensory 

 cells. The wall of the auditory capsule has a second opening 

 (the fenestra rotunda), situated ventrally to the fenestra ovalis. 

 The fenestra rotunda is covered by a membrane separating the 

 perilymph from the tympanic cavity, and its function is to damp 

 down and deaden the vibrations in the perilymph when they 

 reach it. With regard to the auditory vesicle itself, the utricle 

 has the usual three semicircular canals, and the saccule, which 

 is better developed than in lower forms in connexion with the 

 perfecting of the sense of hearing, has a ductus cochlearis. 



The retina of the eye contains mostly cones with very few 

 rods. The lens changes its degree of convexity, and thereby 

 its focal length, as a result of the contraction of the circular 

 iris-muscle. The sclerotic is strengthened by bony plates. 

 Mention has already been made of the three eyelids. The 

 lining of the lids is in places modified into glands. At the 

 inner side of the eye is the Harderian gland which lubricates 

 the " third eyelid " (nictitating membrane) ; at the outer 

 angle is the lachrymal gland. The transparent nictitating 

 membrane, which is really a fold of the conjunctiva, is activated 

 by a muscle derived from the retractor bulbi, and like it 

 innervated by a branch of the abducens. The lower lid is 

 depressed by a special muscle. 



The pineal eye, already seen in Petromyzon, is remarkably 

 developed. Its stalk rises up from the roof of the diencephalon 

 and swells out into a vesicle of which the lower portion forms 

 the sensory layer, and the upper forms the lens. This eye 

 lies below the foramen between the parietals ; it is, however, 

 covered over by connective tissue and a corneoscute. 



The cavity of the nose is enlarged, and a shelf projects 

 inwards from the side wall, increasing the surface of the nasal 

 epithelium and forming a so-called concha. Ventral to the 

 nasal cavities are a pair of pockets, originally formed from 

 the nasal cavities, and lying just above the prevomers. Each 

 opens into the mouth cavity a little way in front of the 



