THE MUSCLES OF THE NECK. 383 
The muscle is covered over by the external pterygoid muscle, by branches of 
the inferior maxillary nerve, and by the internal lateral ligament and ramus of 
the lower jaw. Beneath it are the Eustachian tube, muscles of the soft palate, and 
pharynx (superior constrictor). 
NERVE SUPPLY. 
The inferior maxillary division of the fifth nerve supplies all the muscles of mastication 
except the buccinator, which is supplied by the facial nerve. The internal pterygoid muscle is 
supphed by the nerve before its division into anterior and posterior parts ; the other muscles are 
innervated by the anterior trunk. 
ACTIONS. 
The above muscles, assisted by others in the neck, produce the various movements of the 
lower jaw, as follows :— 
a. Opening and Closure of the Jaw. 
| Weight of the jaw Masseter 
| Digastric 
| Mylo-hyoid Temporal 
Genio-hyoid | 
Genio-hyoglossus Internal pterygoid 
| Infra-hyoid muscles 
Retraction. 
). Protraction and 
External pterygoid | Temporal (posterior fibres) 
Internal pterygoid | 
Temporal (anterior fibres) | 
c. Lateral Movement of the Jaw. 
Internal 35 
External pterygoid | . 
3 of one side 
THE MUSCLES OF THE NECK. 
The muscles in the neck include, in addition to those associated with the muscles 
of the back, and described already (p. 365), the following series :—(1) sterno-cleido- 
mastoid; (2) the muscles of the hyoid bone (supra-hyoid and infra-hyoid) ; (3) the 
muscles of the tongue (extrinsic and intrinsic); (4) the muscles of the pharynx and 
soft palate ; and (5) the prevertebral muscles. 
The sterno-cleido-mastoid muscle is the prominent muscle projecting on 
the side of the neck. It arises by two heads—(1) sterna/, from the anterior surface 
of the manubrium sterni, and (2) clavicular, from the upper surface of the clavicle in 
its inner third. The muscle is inserted into the mastoid process and superior 
curved line of the occipital bone. 
It stretches obliquely across the neck, separating the anterior and posterior 
triangles. Superficially it is crossed by the platysma, the external jugular vein, 
and some of the superficial branches of the cervical plexus. It conceals the splenius 
_ capitis, digastric, levator anguli scapule, scaleni and infra-hyoid muscles, the carotid 
sheath, the cervical plexus, and the spinal accessory nerve. The last-named nerve 
pierces the muscle. 
The sterno-cleido-mastoid muscle is properly divisible into three parts: (1) sterno-mastoid, 
placed superficially, and passing obliquely from the sternum to the mastoid process ; (2) cleido- 
mastoid, placed more deeply, and directed vertically upwards from the clavicle to the mastoid 
process ; and (8) cleido-occipitalis, passing obliquely upwards and backwards behind the cleido- 
mastoid to the superior curved line of the occipital bone. 
The sterno-mastoid muscle is innervated by the spinal accessory nerve, joined by a branch 
from the cervical plexus (C. 2). 
The action of the muscle is referred to below. 
THE MUSCLES OF THE Hyorp BONE. 
The infra-hyoid muscles comprise the omo-hyoid, sterno-hyoid, sterno- 
thyroid, and thyro-hyoid muscles. 
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