HEAD AND ANTERIOR CERVICAL STRUCTURES. 23 



artery is enveloped along with the internal jugular vein and the vagus nerve in a 

 common sheath of fascia. The carotid sheath should be slit open, care being taken 

 of the descendens hypoglossi nerve, which also descends obliquely within it. The 

 intimate connection which this sheath presents with the prevertebral layer of fascia 

 can now be made out. The common carotid usually bifurcates opposite to the 

 upper border of the thyroid cartilage. Observe that the internal carotid at first lies 

 behind, and to the outer side of the external carotid. Upon the coats of these vessels 

 numerous sympathetic twigs ramify, and, at the point of bifurcation of the common 

 carotid, a small oval body, termed the intercarotic body, will be found closely applied 

 to the deep surface of the vessel. The branches of the external carotid which take 

 origin in the area of this triangle run for only a very short part of their course within 

 it. Three branches will be noticed springing from the anterior aspect of the external 

 carotid. Named from below upward these are: (i) the superior thyroid, which, 

 after having given off its hyoid, superior laryngeal, and sterno-mastoid branches, 

 disappears under cover of the omo-hyoid muscle, and enters the inferior carotid 

 triangle; (2) the lingual, which leaves the space by passing under cover of the 

 digastric and stylo-hyoid muscles ; and (3) the facial, which ascends under the 

 same muscles to gain the digastric triangle. The occipital artery commonly springs 

 from the posterior aspect of the external carotid close to the lower border of the 

 digastric, and soon quits the space by running upward and backward under cover 

 of the sterno-mastoid muscle. Its sterno-mastoid branch comes off as it leaves the 

 triangle, and accompanies the spinal accessory nerve. The ascending pharyngeal 

 artery will be found by separating the external and internal arteries from each other, 

 and dissecting between them. It springs from the posterior aspect of the former 

 about half-an-inch above its origin, and then takes a vertical course upward on the 

 prevertebral muscles. 



The internal jugular vein lies close to the outer side of the common carotid 

 artery, and is included within the same fascial sheath. Several tributaries join it as it 

 passes through this space. The most conspicuous of these is the common facial vein, 

 which is formed by the union of the facial vein and the anterior division of the tem- 

 poro-maxillary vein. 



The nerves which are brought into view as the carotid triangle is gradually 

 opened up are very numerous, but they can be classified according to the direction 

 which they take through the space. One large nerve, the hypoglossal, takes a more or 

 less transverse course across the upper part of the space. It forms a loop across this 

 part of the neck immediately below the lower margin of the digastric muscle. Two 

 descend vertically, viz., the vagus and the gangliated cord of the sympathetic. The 

 vagus lies in the posterior part of the carotid sheath between the common carotid 

 artery and the internal jugular vein. The sympathetic cord is imbedded in the poste- 

 rior wall of the carotid sheath. The remaining five nerves traverse the triangle 

 obliquely. Four -run from above downward and forward, viz., the descendens 

 hypoglossi, the thyro-hyoid, the internal and external laryngeal nerves ; and one 

 the spinal accessory is directed from above downward and backward. 



The descendens hypoglossi springs from the hypoglossal nerve as it hooks round 

 the occipital artery and descends within the carotid sheath. It is joined at the upper 

 border of the omo-hyoid by one or two branches from the cervical plexus, and in this 

 manner the ansa hypoglossi is formed. The thyro hyoid is a minute nerve which 

 arises from the hypoglossal nerve before it disappears under cover of the posterior 

 belly of the digastric. The internal laryngeal nerve descends behind the carotid 

 vessels. It will be readily found, with the corresponding artery, in the interval 

 between the hyoid bone and upper border of the thyroid cartilage. It enters the 

 larynx by piercing the thyro-hyoid membrane. The external laryngeal nerve, a 

 branch of the preceding, is a much smaller twig. It leaves the space by passing 

 under cover of the depressor muscles of the larynx and hyoid bone. The spinal ac- 

 cessory nerve is placed high up in the interval between the digastric and the sterno- 

 mastoid muscles. It soon disappears by sinking into the substance of the sterno- 

 mastoid. 



The Submaxillary or Supra-hyoid Triangle may be divided very con- 

 veniently into an anterior and posterior part by the stylo-maxillary ligament and a 



