TKE MANDIBULAR AND IIYOED MUSCLES OF xMAMMALS. 631 
fact : “ Der Scliwnnd des alten M. depressor mandibulae bei 
den Saugern imd dainit der Weclisel in dem Muskelmechanis- 
nuis bei den Oeffnung des Mnndes sind Tatsachen.’^ 
But, perhaps, after all, the muscle has not disappeared. It 
has been shown above that the stapedius muscle of Mammals 
is primarily a levator hyoidei arising from the outer wall of 
the auditory capsule and inserted into the upper part of the 
hyoid bar. Now, the depressor mandibula) of Dipnoi, 
Urodela^ and Sauropsida, as I have shown in a former paper, 
is at first a levator hyoidei inserted into the hyoid bar — a 
condition which persists in Protopterus — and only subse- 
quently becomes attached to the hind end of MeckeFs carti- 
lage. It may be inferred that the stapedius muscle of 
^lammals is homologous with the levator hyoidei of Urodela 
and Sauropsida though not with its later stage of depressor 
mandibulae — there being- no developmental or other evidence 
that it ever gained that secondary attachment. 
The origin of the superficial facial and platysma 
muscles has been investigated by the methods of compara- 
tive anatomy and by direct observation of the phenomena of 
development. 
Eabl stated that the platysma develops in the territory of 
the hyoid arch, grows forward, and upwards behind the 
Anlage of the external ear, and gives rise to the whole of the 
mimetic facial musculature and that of the epicranium. He 
did not further particularise exactly where and how it 
develops, nor the relationship of the Anlage of the platysma 
to the stapedius, posterior digastric and stylo-hyoid. 
Futamura stated that in man and pig the facialis muscu- 
lature is derived from an aggregate of myogenic cells round 
the motor facial nerve. The proximal part consists of two 
la}'ers separated by the N. facialis, of which the lateral 
forms the Platysma colli. 
There is no trace of the superficial facial and platysma 
^ The condition in the Anura is more complicated, the levator 
hyoidei of the tadpole forming only the superficial portion of the 
depressor mandibulse of the adult form. 
VOL. 59 , PART 4 . NEW SERIES. 
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