SUPRA-HYOID REGION. 259 



The Digastric consists of two fleshy bellies united by an intermediate rounded 

 tendon. It is a small muscle, situated below the side of the body of the lower 

 jaw, and extending, in a curved form, from the side of the head to the symphysis 

 of the jaw. The posterior belly, longer than the anterior, arises from the di- 

 gastric groove on the inner side of the mastoid process of the temporal bone, and 

 passes downwards, forwards, and inwards. The anterior belly, being reflected 

 upwards and forwards, is inserted into a depression on the inner side of the lower 

 border of the jaw, close to the symphysis. The tendon of the muscle perforates 

 the Stylo-hyoid, and is held in connection with the side of the body of the hyoid 

 bone by an aponeurotic loop, lined by a synovial membrane. A broad aponeurotic 

 layer is given off* from the tendon of the Digastric on each side, which is attached 

 to the body and great cornu of the hyoid bone : this is termed the supra-hyoid 

 aponeurosis. It forms a strong layer of fascia between the anterior portion of the 

 two muscles, and a firm investment for the other muscles of the supra-hyoid region 

 which lie beneath it. 



The Digastric muscle divides the anterior superior triangle of the neck into two 

 smaller triangles. The upper or submaxillary triangle is bounded, above, by the 

 lower jaw, and mastoid process ; below, by the two bellies of the Digastric muscle: 

 the lower or superior carotid triangle being bounded, above, by the posterior 

 belly of the Digastric ; behind, by the Sterno-mastoid ; below, by the Omo-hyoid. 



Relations. By its superficial surface, with the Platysma, Sterno-mastoid and 

 Trachelo-mastoid, part of the Stylo-hyoid muscle, and the parotid and submaxillary 

 glands. By its deep surface, the anterior belly lies on the Mylo-hyoid ; the pos- 

 terior belly on the Stylo-glossus, Stylo-pharyngeus, and Hyo-glossus muscles, the 

 external carotid and its lingual and facial branches, the internal carotid, internal 

 jugular vein, and hypoglossal nerve. 



The Stylo-hyoid is a small, slender muscle, lying in front of, and above, the 

 posterior belly of the Digastric. It arises from the middle of the outer surface 

 of the styloid process ; and, passing downwards and forwards, is inserted into the 

 body of the hyoid bone, just at its junction with the greater cornu, and immedi- 

 ately above the Omo-hyoid. This muscle is perforated, near its insertion, by the 

 tendon of the Digastric. 



Relations. The same as the posterior belly of the Digastric. 



The Digastric and Stylo-hyoid should be removed, in order to expose the next muscle. 



The Mylo-hyoid is a flat triangular muscle, situated immediately beneath the 

 anterior belly of the Digastric, and forming, with its fellow of the opposite side, 

 a muscular floor for the cavity of the mouth. It arises from the whole length of 

 the mylo-hyoid ridge, from the symphysis in front, to the last molar tooth behind. 

 The posterior fibres pass obliquely forwards, to be inserted into the body of the 

 os hyoides. The middle and anterior fibres are inserted into a median fibrous 

 raphe, where they join at an angle with the fibres of the opposite muscle. This 

 median raphe is sometimes wanting; the muscular fibres of the two sides are then 

 directly continuous with one another. 



Relations. By its cutaneous surface, with the Platysma, the anterior belly .of the 

 Digastric, the supra-hyoid fascia, the submaxillary gland, and submental vessels. 

 By its deep or superior surface, with the Genio-hyoid, part of the Hyo-glossus, and 

 Stylo-glossus muscles, the lingual and gustatory nerves, the sublingual gland, and 

 the buccal mucous membrane, Wharton's duct curves round its posterior border 

 in its passage to the mouth. 



Dissection. The Mylo-hyoid should now be removed, in order to expose the muscles which lie 

 beneath; this is effected by detaching it from its attachments to the hyoid bone and jaw, and 

 separating it by a vertical incision from its fellow of the opposite side. 



The Genio-hyoid is a narrow slender muscle, situated immediately beneath the 

 inner border of the preceding. It arises from the inferior genial tubercle on the 

 inner side of the symphysis of the jaw, and passes downwards and backwards, to 



