The slylo-glossus arises, as a round fleshy bundle, from the tip of the styloid process, and from the adjoining part of the stylo-maxillary ligament, becomes flattened into divergent parts, as it approaches the side of the tongue at the posterior border of the hyo-glossus, after a short course downward, forward, and inward, and is there inserted. A part is continued for some distance along the hyo-glossus, crossing the direction of its fibres, and interwoven with them; other fibres seem to bend into the substance of the tongue, near its base and at right angles to its axis. Its surface corresponds to the parotid gland, external carotid artery, internal pterygoid muscle, and mucous membrane of the mouth; deeply, it lies on the internal carotid artery, the superior constrictor of the pharynx, the tonsil and hyo-glossus.
The gento-hyo-glossus is a large, fan-shaped muscle, radiating from within the symphysis of the jaw to the entire length of the tongue, and constituting, with its fellow, the chief muscular bulk of that fleshy organ. It rises by a strong square mass of short tendinous fibres from the upper genial tubercle, and the fleshy fibres, which succeed, immediately and widely diverge; the highest bend upward and somewhat forward to the tip of the tongue; those, which next follow, occupy its entire remaining length, spreading with more or less obliquity into the substance of the organ, through which on a section they may be followed even to the dorsum: some of these may be traced beyond the posterior edge of the hyo-glossus, expanding on the side of the pharynx just above the hyoid attachment of the middle constrictor, and constituting the so-called lingual origin of the superior constrictor, (see Pharynx); the remaining fibres complete the semicircular spread of the muscle, by passing downward and backward, to be inserted into the upper border of the body of the os hyoides. This muscle is opposed by its entire mesial surface to its fellow: their tubercles of origin are almost blended on the symphysis, and their fleshy fibres are only to be distinguished by a thin intermediate layer of adipose tissue: their upper edges raise the mucous membrane of the mouth on either side of the frænum; their lower edges extend to the hyoid bone in perfect parallelism to each other, and to the genio-hyoidei, which cover them; their outer surfaces, partly covered by the hyo-glossi, form with these on each side the inner wall of a triangular space (roofed by the mucous membrane and floored by the mylo-hyoid muscle) in which lie the terminal branches of the lingual and gustatory nerves, the lingual artery, the sublingual gland, and the excretory duct of the submaxillary.
Close at the implantation of this muscle in the tongue, between its fibres and those of the hyo-glossus, and crossing the direction of both, is a small bundle of fleshy fibres, which runs longitudinally from base to apex, and, since it has no fixed attachment, may most fitly be considered among the intrinsic muscles of the organ; it has been named lingualis. (See Tongue.)
The genio-hyoideus is a strong cylindrical muscle intimately associated with the genio-hyo-glossus, and ordinarily co-operating with its posterior fibres. It rises by a square tendon from the inferior genial tubercle, beside its fellow of the opposite side and just below the genio-hyo-glossus. From this origin it directs itself backward and downward, and is inserted into the upper surface of the body of the os hyoides. Its insertion is somewhat broader than its origin: its inner surface corresponds to that of the opposite side; its upper surface is parallel to the genio-hyo-glossus, which it supports; its under surface rests on the mylo-hyoid, beside its raphe; its outer surface has similar relations to that of the genio-hyo-glossus, contributing with it to form the inner wall of the sub-lingual space just described.
The action of the extrinsic muscles of the tongue is modified and more nicely adapted to the delicate offices of speech by the co-operation of other and intrinsic muscles. These will be described in a future article (see Tongue). Those already considered operate on the tongue en masse;—elevate, advance, depress or retract it, shift its volume to either side, and direct its extremity, by a kind of circumduction, over a wide range of surface. Thus, the stylo-glossus can elevate and retract, the hyo-glossus depress and lateralise; the anterior fibres of the genio-hyo-glossi, with the linguales, regulate the motions of the tip, while the genio-hyoid and adjunct fibres of the genio-hyo-glossi can cooperate in these movements by shifting the base of support in any direction. As the genio-hyo-glossus is of largest bulk, so is it of most various office in the tongue; by its posterior fibres it gives an elevation to the os hyoides by which the tongue is protruded from the mouth; or, half antagonizing this action by its middle fibres, it may so forcibly hollow the dorsum of the tongue as to direct its apex within the incisor teeth, and, with aid of the stylo-glossi, enable it to sweep the concavity of the palate; or, by this co-operating with either hyo-glossus and with the opposite lingualis and stylo-glossus, the tongue may be made, as it were, to probe with its eminently tactile extremity the entire length of the alveolar arches, or by a yet more definite contraction to exert suction on any spot with which its dorsum can have contact.
The third class includes the sterno-cleido-mastoideus and the platysma myoides.
The sterno-cleido-mastoideus is a long and powerful muscle, obliquely crossing the side of the neck, from the neighbourhood of the sterno-clavicular joint to the mastoid process of the temporal bone. It is fleshy in almost its whole extent; flattened at the extremities, but rather prismatic in the intermediate portion; and the anterior edge, which is more particularly continuous with the sternal origin of the muscle, and which, in certain positions of the neck, raises the integuments in a well-known diagonal relief, considerably exceeds the thickness of the posterior border. The name of the muscle is a summary of its attachments. It arises by two heads, which are usually
