THE NERVOUS SYSTEM. 237 



between the cornea and crystalline lens. It always 

 escapes as soon as the cornea is punctured. The vitreous 

 humor is of a jelly-like consistency, filling the large pos- 

 terior chamber caudad of the lens. It is perfectly trans- 

 parent and is surrounded by a delicate capsule, the 

 hyaline membrane. 



The crystalline lens is a transparent biconvex tissue 

 having a vertical diameter of about one centimeter 

 and a shorter diameter through its optical axis. It 

 is enclosed in a transparent elastic capsule, some of 

 whose fibers are continued peripherad as the suspensory 

 ligament which is inserted in the choroid coat (Fig. 106). 



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 

 auditorius meatus externus. The pinna is the pro- 

 jecting portion of the ear capable of being moved by 

 muscles, and is composed of integument strengthened 

 by fibrocartilage. The auditorius meatus externus ex- 

 tends from the base of the pinna to the tympanic mem- 

 brane (Fig. 107). Its outer or lateral third is formed 

 by cartilage, and the remainder by the tympanic por- 

 tion of the temporal bone (Fig. 17). The meatus is 

 lined with mucous membrane in which are numerous 

 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. 107). It is separated 

 from the external auditory meatus by the delicate 

 translucent membrane, the membrana tympani. The 

 petrous bone containing the internal ear forms part of 

 the inner or nasal wall. In the petrous bone are two 

 foramina which may be seen in a dry skull by looking 



