DISSECTION OF THE NECK. 83 



nerves ; and is placed across the anterior triangular space of the neck in 

 the position of a line from the mastoid process to a little above the hyoid 

 bone : along its lower border lie the occipital artery and the hypoglossal 

 nerve ; the former passing backwards, the latter forwards. The anterior 

 belly rests on the mylo-hyoid muscle. 



The muscle forms the lower boundary of a space between the jaw and 

 the base of the skull, which is subdivided into two by the stylo-maxillary 

 ligament. In the posterior portion are contained the parotid gland ( 10 ), 

 and the vessels and nerves in connection with it (p. 41) ; in the anterior, 

 are the submaxillary gland ("), with the facial and submental vessels, and 

 deeper still, the muscles between the chin and the hyoid bone. 



Action. The lower jaw being movable, the muscle depresses that bone 

 and opens the mouth. If the jaw is fixed, the two bellies acting will ele- 

 vate the hyoid bone. 



It is supposed that the posterior belly may assist in moving back the 

 head when the jaw is fixed. 



The STYLO-HYOID MUSCLE is thin and slender, and has the same posi- 

 tion as the posterior belly of the digastric. It arises from the outer sur- 

 face of the styloid process, near the base, and is inserted into the body of 

 the os hyoides. 



The muscle has the same connections as the posterior belly of the digas- 

 tric ; and its fleshy fibres are usually perforated by the tendon of that 

 muscle. In some bodies the stylo-hyoideus is absent. 



Action. This muscle elevates the os hyoides preparatory to swallowing, 

 and checks, with the posterior belfy of the digastric, the too forward move- 

 ment of that bone by the other elevators. 



The HYPOGLOSSAL NERVE (ninth cranial) appears in the anterior tri- 

 angle at the lower edge of the digastric muscle, where it hooks round the 

 occipital artery; it is then directed forwards to the tongue below that 

 muscle, and disappears in front beneath the mylo-liyoideus. In this course 

 the nerve passes over the two carotids ; and near the cornu of the os 

 hyoides it crosses also the lingual artery, so as to become higher than the 

 vessel. From this part arise the descendens noni branch, and a small 

 muscular offset to the thryo-hyoideus. 



The descending branch (ram. descend, noni) leaves the trunk of the 

 hypoglossal on the outer side of the carotid artery, and descends on the 

 front of (sometimes in) the sheath of the vessel to about the middle of the 

 neck, where it is joined by the communicating branches of the cervical 

 nerves. After the union of the spinal nerves, offsets are supplied to the 

 depressor muscles o 1 ' the os hyoides, viz., omo-hyoid (both bellies), sterno- 

 hyoid, and sterno-thyroid : sometimes another offset is continued to the 

 thorax, where it joins the phrenic and cardiac nerves. 



The connection between the descendens noni and the spinal nerves is 

 formed by two or more cross filaments, so as to construct an arch with the 

 concavity up\vards ; and an interchange of fibrils between the two nerves 

 is supposed to take place. 



The EXTERNAL CAROTID ARTERY (fig. 17, d) springs from the bifurca- 

 tion of the common carotid at the upper border of the thyroid cartilage, 

 and furnishes branches to the neck, the face, and the outer parts of the 

 head. 



From the place of origin it ascends in front of the mastoid process, and 

 ends near the condyle of the jaw in the internal maxillary and temporal 

 branches. The artery lies at first to the inner side of the internal carotid, but 



