MYODOME AND TRIGEMINO-FACIALIS CHAMBER 299 
gemino-facialis chamber, the tympanic cavity of mammals thus 
being a derivative of this chamber of fishes. The tegmen tym- 
pani and the malleus, incus, and stapes are then quite cer- 
tainly parts of the outer wall of this cavity, and hence derived 
from the quadrate, this being as Driiner (04) has maintained 
for the malleus, incus, and stapes. It would also seem as if the 
annulus tympanicus must have the same origin, thus complet- 
ing the outer wall of the cavity and encircling the part that was 
broken up to form the auditory ossicles. The fact that the 
stapes may be traversed by the arteria stapedialis is in ac- 
cord with the perforation, in Amia, of the lateral wall of the 
trigemino-facialis chamber by the external carotid.' 
The tympanic cavity is traversed, in mammals, by the chorda 
tympani, and Jacobson’s nerve and sympathetic fibers enter 
it. In fishes the pars jugularis of the trigemino-facialis chamber 
is traversed by a sympathetic nerve and frequently (always ?) 
also by a communicating branch from the nervus facialis to the 
nervus trigeminus, and Jacobson’s nerve enters it as a part of 
the truncus facialis. The communicating branch from the ner- 
vus facialis to the nervus trigeminus must then be the chorda 
tympani, and that nerve must be a prespiracular one, for in 
fishes it certainly is prespiracular. The chorda tympani must 
then be represented, in fishes, in the ramus mandibularis inter- 
nus trigemini of my descriptions of Amia (Allis, ’01, p. 188). 
In fishes the spiracular canal or a diverticulum of it may lie 
along the lateral wall of the trigemino-facialis chamber. If 
a diverticulum of either of those canals were to expand into the 
pars jugularis of the chamber, it would evidently give rise to a 
tympanic cavity connected with the pharynx by an eustachian 
tube, or the same result would be obtained by the expansion in- 
‘Later work has somewhat modified this opinion and convinced me that the 
incus, alone, corresponds to the lateral wall of the trigemine-facialis chamber 
of fishes, both structures being derived from the posterior branchial-ray bar of 
the mandibular arch. The malleus and the teleostean quadrate both represent 
the epal element of the mandibular arch. The styloid and mastoid processes 
are, respectively, the anterior and posterior branchial-ray bars of the hyal arch, 
and the stapes probably the pharyngohyal. The chorda tympani is a posttre- 
matic nerve. 
