The Head and Neck. 161 



dorsal surface of the common carotid on the medial side of 

 the sympathetic trunk. It arises at the level of the posterior 

 margin of the thyreoid cartilage, 

 (s) The third and fourth cervical nerves may be traced from their 

 origin in the intervertebral foramina to the musculature of 

 the neck. They encircle the basioclavicularis muscle, under 

 cover of the sternomastoideus and cleidomastoideus. 



5. Dissection of the muscles of mastication and related structures of 

 the mandible. 



On account of the narrowness of the space lying between the two limbs of the 

 mandible, and the great depth of its angle, it is necessary, in order to expose the 

 surface for the deep dissection of the ventral portion of the head and neck, to 

 remove one-half of the mandible entirely. Hence the following order: 



(a) The masseter muscle. Origin: The entire surface of the 

 zygomatic arch; tendinous from its anterior angle (spina 

 masseterica). Insertion: Lateral surface of the angle of the 

 mandible (1, a). 



■The orbital structures should be freed from the zygomatic arch by 

 passing a knife along its dorsal margin. The zygomatic arch may then 

 be divided anteriorly and posteriorly and removed, together with the 

 " whole insertion of the masseter muscle. 



(b) The temporalis is a slender, somewhat triangular muscle 

 arising from the reduced temporal fossa (sulcus temporalis) 

 of the skull and inserted by a long stout tendon on the 

 medial side of the reduced coronoid process. The muscle 

 may be exposed by dividing the temporal portion of the 

 posterior supraorbital ligament which holds its tendon in 

 place; then divided. 



To remove one-half of the mandible : 



Divide the mandibular symphysis, and pass a knife along the 

 medial surface of the side to be removed. The tip of the 

 knife must be kept close to the bone, so that the underlying 

 soft parts, except for being divided, will be kept uninjured. 

 The medial surface of the mandible should be clean when 

 removed. The bone may be turned laterad and detached 

 from the skull at the articulation. 



The structures appearing on the cut surface include the insertion 

 of the digastricus and the margin of the mylohyoideus ; also : 



(c) The pterygoideus internus muscle. Origin: Pterygoid pro- 

 cess of the skull. Insertion: Ventral portion of the medial 

 surface of the angle. 



(d) The pterygoideus externus. Origin: Lateral plate of the 

 pterygoid process. Insertion : Dorsal portion of the medial 

 surface of the angle. Both muscles are strongly de- 

 veloped. 



(e) The inferior alveolar artery (a. alveolaris inferior) lies be- 

 tween the two pterygoidei. Its cut end marks the point of 

 entrance into the mandible through the mandibular foramen. 



