224 ELEMENTS OF MAMMALIAN ANATOMY. 



crystalline lens. It always escapes as soon as the cornea 

 is punctured. The vitreous humor is of a jelly-like con- 

 sistency, filling the part of the eyeball caudad of the lens. 

 It is perfectly transparent and is surrounded by a delicate 

 capsule, the hyaline membrane. 



The crystalline lens is a transparent biconvex tissue hav- 

 ing a vertical diameter of about one centimeter and a shorter 

 diameter through its optical axis. It is enclosed in a trans- 

 parent elastic capsule, some of whose fibers are continued 

 peripherad as the suspensory ligament which is inserted in 

 the choroid coat (Fig. no). 



The Auditory Organ. The organ of hearing is com- 

 posed of three parts the external ear, middle ear, and 

 internal ear. The first consists of the pinna and the audi- 

 torius meatus externus. The pinna is the projecting por- 

 tion of the ear capable of being moved by muscles, and is 

 composed of integument strengthened by fibrocartilage. 

 The audit onus meatus externus extends from the base of 

 the pinna to the tympanic membrane (Fig. 1 1 1 ) . Its outer 

 or lateral third is formed by cartilage, and the remainder by 

 the tympanic portion of the temporal bone (Fig. 17). The 

 meatus is lined with mucous membrane in which are numer- 

 ous sebaceous and oleaginous glands. The latter secrete 

 the wax of the ear. 



The middle ear, or tympanum,, is an irregular cavity about 

 one centimeter in diameter contained in the lateral chamber 

 of the bulla (Fig. in). It is separated from the external 

 auditory meatus by the delicate translucent membrane, the 

 membrana tympani. The petrous bone containing the in- 

 ternal ear forms part of the inner or medial wall. In the 

 petrous bone are two foramina which may be seen in a dry 

 skull by looking through the external auditory meatus. 

 The more dorsal foramen is the fenestra ovalis, which in 

 the recent state is closed by a membrane to which the foot 



