286 THE MUSCLES. 



sphenoid bone and the suhsphenoidal process, and are directed backwards and 

 upwards to be fixed to the neck of the inferior maxillary condyle. 



Relations. — Outwardly, with the orbital fasciculus of the temporalis muscle 

 and the temporo-maxillary articulation. Inwardly, with the numerous nerves 

 emanating from the inferior maxillary branch, and with the internal pterygoid 

 and tensors palati. 



Action. — When the two external pterygoids act in concert, the inferior 

 maxilla is pulled forward ; but if only one contract, the propulsion is accompanied 

 by a lateral movement, during which the extremity of the jaw is carried to the 

 opposite side. 



5. DiGASTRICUS (StYLO-MAXILLARIS). 



Synonyms. — Bourgelat has made two distinct muscles of this — the digastricus and stylo- 

 maxillaris. Girard lias described it as the stylo-maxillaris. (Percivall has evidently followed 

 Bourgelat's example, and divided the muscle into digastricus and stylo-maxillaris. Leyh 

 adopts the same course.) 



Form — Structure — Situation — Direction. — Composed of two fleshy bodies 

 more or less divided by intersections, and united at their extremities by a median 

 tendon, this muscle is situated in the intermaxillary space, and extends from the 

 occiput to near the symphysis of the chin, describing a curve upwards. 



Attachments. — It takes its origin from the styloid process of the occipital 

 bone, by its superior fleshy body. It terminates : 1. On the curved portion of 

 the posterior border of the lower jaw by a considerable fasciculus, which is 

 detached from the superior fleshy body.^ 2. On the internal face of the same 

 bone and the straight portion of its posterior border, by aponeurotic digitations 

 which succeed the muscular fibres of the inferior fleshy body. 



Relations.— The superior belly of the muscle responds, superficially, to the 

 parotid gland and the tendon of insertion of the sterno-maxillaris ; deeply, to 

 the guttural pouch, the submaxillary gland, and the larynx and pharynx. The 

 median tendon passes through the ring of the hyoideus magnus. The lower 

 belly is in contact, outwards, with the ramus of the inferior maxilla ; inwards, 

 with the mylo-hyoideus muscle. 



Action. — When this muscle contracts, it acts at the same time on the hyoid 

 bone, which it raises in becoming straight, and on the lower jaw, which it pulk 

 backwards and depresses at the same time. 



E. Hyoideal Region. 



This region includes six muscles grouped around the os hyoides, which they 

 move. Five of these are pairs : the mylo-hyoideus, genio-hyoideus, stylo-hyoideus, 

 kerato-hyoideus {hyoideus parvus), and the occipito-styloideus. The single one is 

 the hyoideus transversus. 



Prepara(/on.— Separate the head from the trunk, and remove the muscles of the cheeks on 

 one side, with the parutid gland. 2. The branch of the inferior maxilla being thus exposed, it 

 is sawn through in two places; at first beliind the last molar, then in front of the first. 3. 

 After having separated the pterygoids and the stylo-maxillaris from the upper fragment or 

 condyle, and the coronoid process, it is torn off by pulling it backwards ; then the pterygoids 

 and digastricus are excised. 4. The inferior fragment of the jaw bearing the molar teeth is 



1 This is the fasciculus which Bourgelat has described as a distinct muscle, and named the 

 ttylo-maxillaris. 



