344 THE MUSCULATURE 



of the digastric and of the stylo-hyoid is from the seventh cranial nerve, the 

 sensor}' innervation probably from the glosso-pharyngeal cranial nerve. The 

 mylo-hyoid and the anterior belly of the digastric are supplied by the masticator 

 (fifth) cranial nerve; the genio-hyoid from the hypoglossal by a branch, the 

 fibres of which are possibly derived through anastomosis from the first cervical 

 nerve. 



From the morphological standpoint, therefore, the stylo-hyoid and the posterior belly of 

 the digastric belong to the facialis group; the anterior belly of the digastric and the mylo-hyoid 

 to the group of mandibular muscles, and the genio-hyoid to the muscles of the tongue inner- 

 vated b}^ the hypoglossal, or, if we consider the nerve-fibres of the nerve to the genio-hyoid as 

 derived from the first cervical nerve, to the same group as the infra-hyoid muscles. It is con- 

 venient, however, to follow the usual custom of considering these muscles as a suprahyoid group. 



FASCIA 



The muscles of this group Ue internal to that portion of the external cervical fascia which 

 extends above the hyoid bone. This fascia, which is described on p. 347, comes into contact 

 merely with the tendon, the anterior belly, and to a slight extent with the posterior belly of the 

 digastric muscle. Above the tendon it sends inward a process which curves down internal to 

 the tendon, and is inserted into the external surface of the hyoid bone. The individual muscles 

 of the group are covered by delicate adherent membranes. An aponeurotic membrane usually 

 extends between the anterior bellies of the digastric muscles of each side. 



MUSCLES 



(Fig. 348) 



The stylo-hyoideus. — Origin. — From the lateral and dorsal part of the base of the styloid 

 process by a rounded tendon which soon becomes a hollow cone to the internal surface of which 

 the fibre-bundles of the muscle are attached. Structure and Insertion. — The fibre-bundles are 

 inserted on both sides of a slender tendon which divides to let the tendon of the digastric pass 

 through and then is attached to the ventral surface of the body of the hyoid bone near its junc- 

 tion with the great cornu. 



Nerve-supply. — From the facial nerve as it emerges from the stylo-mastoid foramen a small 



twig is given off which enters the proximal third of the deep surface of the muscle. The glosso- 

 pharyngeal nerve also gives to it a small twig, probably sensory. 



Relations. — It descends immediately in front of the posterior belly of the digastric. Ex- 

 ternally lie the parotid and submaxillary glands. Medially it crosses the internal and external 

 carotid artery, the hypoglossal nerve, the stylo-pharyngeus muscle, the superior constrictor 

 of the pharynx, and the hyo-glossus muscle. The posterior auricular artery passes between it 

 and the posterior belly of the digastric and the external maxillary artex'y crosses over it. 



The digastricus. — The posterior belly arises by tendinous processes from the mastoid 

 (digastric) notch of the temporal bone. The fibre-bundles form a ribbon-like belly which con- 

 verges on the intermediate tendon. This begins as a semiconical laminar i)rocess on the outer 

 surface of the muscle a short distance above the hyoid bone. The anterior belly arises by short 

 tendinous processes from the digastric fossa of the mandible. This attachment is often de- 

 scribed a.s an insertion. The fibres converge on both surfaces of the flattened anterior end of 

 the intermediate tendon. The intermediate tendon lies a variable distance above the hyoid 

 bone, usually less than a centimetre. It curves upward toward each belly of the muscle. It 

 is united to the outer surface of the body and to the base of the great cornu of the hyoid bone 

 by an aponeurotic expansion from its inferior margin. Other expansions arc usually continued 

 into the interdigastric aponeurotic membrane. Occasionally the intermediate tendon of the 

 digastric is bound to the hyoid hone by a fibrous loop which allows the tendon free play. 



Nerve-suppli/. — The facial nerve near the stylo-mastoid foramen gives off a branch which 

 enters the proximal third of the anterior margin of the muscle. From this a ramus may be 

 continued through the muscle to the; glosso-pharyngeal nerve. The anterior belly is supplied 

 by a branch of the nerve to the mylo-liyoid muscle. This enters the middle of the lateral part 

 of the deep surface. Very rarely the vagus may sui)ply the anterior belly, the hypoglossal, 

 the posterior belly. 



Relations. — The posterior belly of the digastric lies internal to the mastoid process and 

 the longissimus capitis (trachelo-mastoid), splenius, and sterno-cleido-mastoid muscles. Pos- 

 teriorly near its origin are the rectus capitis lateralis and oblifjuus cap. sup. muscles, the occip- 

 ital artery and the spinal accessory nerve. It helps to form the deep wall of the cavity in which 

 the parotid gland is placed. Internally it crosses the origin of the styloid muscles, the carotid 

 arteries, the internal jugular vein, and the twelfth craiiiaJ nerve. The intermediate tendon 

 of insertion lies below the inferior margin of the submaxillary gland, and crosses the hyo- 

 glossus and mylo-hyoid muscles. The relations to the stylo-hyoid muscle have been described 

 above. The anterior belly lies on the mylo-hyoid and is covered by the external cervical fascia 

 and the platysnia. 



The mylo-hyoldeus. — Origin. — From the mylo-hyoid ridge of the mandible. Structure 

 and Insertion. — Its fibre-bundles take an oblique course and are inserted into — (1) a median 

 raphe extending from the middle of the ventral surface of the hyoid bone nearly or quite to the 



