94 
MAMMALIAN ANATOMY 
muscle, which arises from the posterior wall of the tympanum and 
is inserted into the stapes. 
The finer details of the internal ear cannot be made out from 
such material as this, but much of the general topography can be 
determined. Starting with the internal acoustic meatus, note that 
both the acoustic and facial nerve enter the petrous portion of the 
bone. By chipping away the surface of the bone with the bone 
forceps follow the course of the facial nerve along the line bounding 
middle and internal ear, and note that the acoustic nerve, which 
is more deeply located, has a ganglion within the acoustic meatus, 
beyond which point its two divisions diverge, the cochlearis 
passing forward and the vestibularis downward and backward. 
By further chipping away the bone certain portions of the laby¬ 
rinth itself may be located. 
The labyrinth consists of a vestibule, which contains two 
distended portions of the membranous labyrinth known as the 
saccule and the utricle, connected with each other through the 
endolymphatic duct. From the vestibule leads (i) the very 
elaborate coiled structure, known as the cochlea, located ventral 
and anterior to it, and (2) the three delicate semicircular ducts-, 
located dorsal and posterior to it (cf. dissection of the dogfish 
showing these canals). The membranous portion of the cochlea 
connects with the cavity of the saccule through a slender duct 
known as the ductus reuniens. Within the lining of the vestibule 
and the cochlea are the receptive organs of the sense of hearing, the 
spiral organ of Corti (cf. p. 65), while the semicircular canals are 
the organs of equilibration. 
Show by diagram the relation of the parts of the ear. 
F. THE FLOOR OF THE MOUTH, AND THE TONGUE. 
With the aid of a saw remove from those specimens of sheep’s 
heads which were not sawn sagittally, all the remaining portion 
dorsal to the surface of the tongue, and study the tongue and 
floor of the mouth. Use for comparison, the previous study of the 
median sagittal section of the head, also the interior of your 
own mouth, viewed with a mirror, or the interior of the mouth of a 
fellow-student. Compare, further, with demonstration dissections 
and models of the human tongue and larynx. 
