THE SUPEAHY01D MUSCLES. 301 



The omo-hyoid and sterno-hyoid are portions of the same muscular sheet, the original 

 continuity of the two being indicated by the fascia which binds down the posterior belly of 

 the omo-hyoid to the clavicle, and which has been seen in the infant to contain striped 

 muscular fibres. The chief varieties of the omo-hyoid may be explained as resulting from an 

 increased or diminished degree of cleavage in this sheet, or the reappearance (or persistence) 

 of the part which is normally suppressed. (Gregenbaur, " Ueber den Musculus Omo-hyoideus. 

 und seine Schlusselbeinverbindung," Morph. Jahrb.. i, 243.) 



Nerves. The infrahyoid muscles receive their nervous supply from the first three cervical 

 nerves, by means of fibres which run in great part through the hypoglossal trunk. Thus, 

 from the first two cervical nerves fibres pass to the hypoglossal, which they leave mainly as. 

 the descending branch, but some are continued on to form a special branch to the Irhyro-hyoid. 

 The descending branch is joined by communicating branches from the second and third 

 cervical nerves, and from the loop thus constituted offsets proceed to the sterno-hyoid, sterno- 

 thyroid and both bellies of the omo-hyoid. 



THE SUPRAHYOID MUSCLES. 



The digastric muscle consists of two elongated fleshy bellies united by an inter- 

 vening rounded tendon. The posterior belly arises from the digastric fossa of the 

 temporal bone, and is directed downwards, forwards and inwards, tapering gradually, 

 towards the hyoid bone : the anterior is attached to a rough depression on the 

 lower border of the lower jaw, close to the symphysis ; it is shorter and broader than 

 the posterior belly, and is directed downwards, backwards, and slightly outwards. 

 The intervening tendon is connected to the body and great cornu of the hyoid 

 bone by a broad band of aponeuro tic fibres and by the fleshy fibres of the stylo- 

 hyoid muscle, through which the tendon passes. 



Relations. The posterior belly is for the most part concealed by the mastoid process, the 

 sterno-mastoid and splenius muscles, and the parotid gland ; it crosses the internal and 

 external carotid arteries, with the internal jugular vein and accompanying nerves. The 

 anterior belly is placed immediately beneath the deep cervical fascia, and rests on the mylo- 

 hyoid muscle ; its inner border is connected by a dense aponeurosis with its fellow of the 

 opposite side. 



Varieties. The digastric muscle is subject to many variations. The posterior belly has. 

 been seen receiving an accessory slip from the styloid process, or arising entirely from that 

 part, or connected by a slip with the middle or inferior constrictor of the pharynx. The 

 anterior belly has been joined by a slip arising from the lower jaw in front of the angle. In 

 rare cases the muscle is monogastric, the anterior attachment in that case taking place about 

 the middle of the body of the lower jaw. The anterior belly is frequently divided into two or 

 more parts, one or even two of which may cross the middle line and decussate with similar 

 slips from the muscle of the opposite side ; or a slip sometimes passes to the median raphe of 

 the mylo-hyoid, or becomes incorporated with its fellow of the opposite side. Its deep 

 surface is sometimes united by muscular fibres with the subjacent mylo-hyoid. The tendon of 

 the digastric has been seen in front of, or more rarely behind the stylo-hyoid, instead of 

 passing through it. 



The mento-hyoid is an occasional median slip found passing from the body of the hyoid 

 bone to the chin ; it sometimes consists of two parallel bands. 



The stylo-hyoid is a slender muscle placed along the upper border of the posterior 

 belly of the digastric. It arises by a narrow tendon from the back of the styloid 

 process of the temporal bone near the root, and inclines downwards and forwards, 

 to be inserted into the hyoid bone at the union of the great cornu with the body. 

 Its fibres are usually divided into two fasciculi near its insertion, for the trans- 

 mission of the tendon of the digastric muscle. 



Relations The upper part of the stylo-hyoid is covered by the parotid gland ; the lower 

 part is superficial. The muscle crosses the external carotid artery. The lower part of the 

 stylo-hyoid, the anterior belly of the digastric, and the base of the lower jaw bound a 

 triangular space which contains the submaxillary gland, and is known as the submaxillary 

 triangle. 



Varieties. The stylo-hyoid is not unfrequently wanting ; while, on the other hand r 

 doubling of the muscle has also been recorded. It is occasionally placed on the inner side of 



