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. r10). 
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 auditorius meatus externus extends from the base of 
the pinna to the tympanic membrane (Fig. 111). 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. t11). 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 
