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DIGASTRIOUS. 31
bellies of the Sterno-Hyoidei, and of the other lower muscles,
are by much the longest ; but on the contrary, we find that the
Os Hyoides, with the Thyroide Cartilage, is a little raised in
the depression of the Jaw, which we may suppose to be done
by the anterior belly of the Digastric : (i) and secondly, if these
aponeurosis, which comes off from the round tendon of the muscle, and
is in part united to the os hyoides and in part joins the aponeurosis of
the opposite side. He denies that there is anything like a sheath in
which the middle tendon slides. Lastly, he describes some experiments
on the living and dead subject, which he believes disprove the agency,
at least, to any great extent, of the anterior or posterior belly of the
digastricus in depressing the jaw. He believes the action of the muscle
to be principally exercised in deglutition, in which act it pulls the os
hyoides upwards, so as to press the root of the tongue against the soft
palate. The real depressors of the maxilla he considers to be the
sterno-hyoidei and genio-hyoidei, together with the coraco-hyoidei and
mylo-hyoidei, which may be looked upon as digastrics whose middle
intersection is at the hyoid bone ; and also the sterno-thyroidei, thyro-
hyoidei, hyo-glossi, and genio-glossi, which, in their united action, are
to be considered as two many-bellied muscles acting on the lower
jaw. (1)
The opinion which is now generally held, and which is probably
correct, may be taken as a mean between the opposite views of Hunter
and Monro. Anatomists generally allow that the principal instrument
in the depression of the jaw is the digastric ; but in order to the pro-
duction of that effect it is necessary that the os hyoides should be fixed,
which is done by the agency of the muscles that pass between that bone
and the sternum and scapula, so that the latter become indirectly
depressors of the maxilla. It is also generally admitted that all the other
muscles which pass from the os hyoides to the maxilla when the former
is fixed, may be accessories of the digastric in this act. When, on the
contrary, the jaw is fixed, the contraction of the digastric must raise the
os hyoides and the parts attached to it. This action of the digastric is
called into play in deglutition when, together with the stylo-hyoid,
mylo-hyoid, and genio-hyoid, it elevates the hyoid bone, and with it the
base of the tongue. In swallowing the mouth is first closed.]
(i) [Monro states that the os hyoides moves downwards and forwards
when the mouth is opened. In observations on myself and others I
have never been able to arrive at the conviction that the hyoid bone
and thyroid cartilage are raised when the lower jaw is depressed, as is
asserted in the text.]
(1) Medical Essays and Observations by a Society in Edinburgh, vol. i.,
1733.