162 DIGASTKICUS STYLO-HYO1DEUS. 



Dissection. These are best dissected by placing a high block be- 

 neath the neck, and throwing the head backwards. The integument 

 has been already dissected away, and the removal of the cellular 

 tissue and fat brings them clearly into view. 



The digastricus (Sis, twice, yatfr^ belly) is a small muscle situated 

 immediately beneath the side of the body of the lower jaw ; it is 

 fleshy at each extremity, and tendinous in the middle. It arises 

 from the digastric fossa, upon the inner side of the mastoid process 

 of the temporal bone, and is inserted into a depression on the inner 

 side of the lower jaw, close to the symphysis. The middle tendon 

 is held in connexion with the body of the os hyoides by an aponeu- 

 rotic loop, through which it plays as through a pulley; the loop 

 being lubricated by a synovial membrane. A thin layer of aponeu- 

 rosis is given off from the tendon of the digastricus at each side, 

 which is connected with the body of the os hyoides and forms a 

 strong plane of fascia between the anterior portions of the two mus- 

 cles. This fascia is named the supra-hyoidean. 



Relations. By its superficial surface with the platysma myoides, 

 the sterno-mastoid, the anterior fasciculus of the stylo-hyoid muscle, 

 the parotid gland, and submaxillary gland. By its deep surface 

 with the styloid muscles, the hyo-glossus, the mylo-hyoid muscle, 

 the external carotid artery, the lingual and the facial arteries, the 

 internal carotid artery, the jugular vein, and the hypoglossal nerve. 

 The digastric muscle forms the two inferior boundaries of the sub- 

 maxillary triangle, the superior boundary being the side of the body 

 of the lower jaw. In the posterior half of the submaxillary triangle 

 is situated the submaxillary gland and the facial artery. 



The stylo-hyoideus is a small and slender muscle situated in 

 immediate relation with the posterior belly of the digastricus 

 muscle, being pierced by its tendon. It arises from the middle of 

 the styloid process, and is inserted into the body of the os hyoides 

 near to the middle line. 



Relations. By its superficial surface with the posterior belly of 

 the digastricus, the parotid gland and submaxillary gland ; its 

 deep relations are similar to those of the posterior belly of the 

 digastricus. 



The digastricus and stylo-hyoideus must be removed from their 

 connexion with the lower jaw and os hyoides, and turned aside in 

 order to see the next muscle. 



The mylo-kyoideus (f^Xr), mola, i. e. attached to the molar ridge 

 of the lower jaw) is a broad triangular plane of muscular fibres, 

 forming, with its fellow of the opposite side, the inferior wall or 

 floor of the mouth. It arises from the molar ridge on the lower 

 jaw, and proceeds obliquely inwards to be inserted into the raphe 

 of the two muscles and into the body of the os hyoides ; the raphe 

 is sometimes deficient at its anterior part. 



Relations. By its superficial or inferior surface, with the pla- 

 tysma myoides, the digastricus, the supra-hyoidean fascia, the sub- 

 maxillary gland and the submental artery. By its deep or superior 



