CONTRIBUTIONS TO ZOOLOGICAL ANATOMY. 591 
lower jaw, and is formed of a broad triangular plane of muscular 
fibres, which, with the muscle of the opposke side, fills up the 
intermaxillary space, and forms a floor for the inferior part of the 
cavity of the mouth. It arises from the rounded ridge that runs 
along the inside of the lower jaw from the symphysis backwards to 
a line with the anterior edge of the ramus, where it turns upwards 
along the roughened impressions for the pterygoideus interims 
muscle, to the posterior part of the alveolar edge of the last molar 
tooth. The fibres from these points run obliquely inwards and 
backwards, and become inserted into the raphe formed by the two 
muscles, and into the whole of the projecting carinated process of 
the body, and into part of the posterior cornu of the os hyoides. 
Interiorly, this muscle is related to the anterior belly of the digas- 
tricus and to the submaxillary gland and vessels; behind, its 
tendinous fibres join with the insertion of the sterno-hyoid and 
thyro-hyoid ; and superiorly with the genio-hyoid and all the in- 
trinsic muscles of the tongue ; and at the inferior edge of the hyo- 
glossus it forms a concavity for the reception of the sublingual 
gland and gustatory branch of the fifth pair of nerves, and which 
separate it from the mucous membrane of the mouth. Its action 
on the os hyoides is similar to the two former muscles; but it also 
aids materially in the evacuation of the saliva from both the sub- 
maxillary and sublingual glands ; — the duct of the former gland, 
twining around the posterior border of the mylo-hyoid muscle, 
becomes compressed between .it and the hyo-glossus in its course 
to the mucous surface of the mouth. 
The Genio-hyoideus is placed immediately above the mylo- 
liyoideus, and on either side of the mesial plane of the mouth. It 
arises , with its fellow of the opposite side, by a strong and rounded 
tendon, from the concavity of the symphysis menti, and, extending 
downwards and backwards, forms a rounded fleshy belly, that 
becomes inserted into the apex of the carinated process of the 
body of the os hyoides. Interiorly and externally it lies on the 
mylo-hyoideus ; and superiorly and externally it is related to the 
genio-hyo-glossus and the sublingual gland, the stylo-glossus longus 
and brevis, and the hyo-glossus muscles. Its action is to elevate 
the os hyoides, and to compress the sublingual gland against the 
side of the tongue. 
Second Group . — The Intrinsic Muscles of the Os Hyoides. 
Under this group I have included two muscles that are situated 
between the cornua of the os hyoides, and act especially on its 
anterior or moveable cornua. These muscles are, the 
Anterior chondro-hyoideus. 
Posterior choudro-hvoideus. 
