THE HEAD AND NECK 1129 



and the inferior thyroid arteries. Posterior. — The vertebral column 

 and the longus colli muscles, covered by the prevertebral fascia. 

 Lateral. — On either side there is the carotid sheath with its 

 contents. The right and left recurrent laryngeal nerv-es are inti- 

 mately related to the oesophagus, and ascend on each side in the 

 groove between it and the trachea. 



The cervical part of the oesophagus receives its blood-supply from 

 the inferior thyroid arteries, right and left, each of which accompanies 

 the corresponding recurrent laryngeal nerve. 



Inferior Thyroid Lympliatic Glands. — These glands he in the 

 groove between the cer\'ical parts of the trachea and oesophagus, 

 along the course of the inferior thyroid artery and recurrent laryn- 

 geal nerve. Their afferent vessels are derived from (i) the lower 

 part of the lateral lobe of the thyroid body, and (2) the adjacent 

 parts of the trachea and oesophagus. Their efferent vessels pass to 

 the inferior deep cervical glands. 



For a description of the oesophagus in the thorax, including its 

 structure and development, see Index. 



Suprahyoid Region — Muscles — Digastric. — The digastric muscle 

 consists of two beUies, posterior and anterior. Origin. — The 

 posterior belly arises from the digastric groove on the inner aspect 

 of the mastoid part of the temporal bone; and the anterior belly 

 arises from the digastric impression on the inner surface of the 

 body of the inferior maxilla, close to the base and sjmiphysis. 



Insertion. — The two bellies end upon an intervening tendon, 

 about 2 inches long, which is inserted by means of a broad fibrous 

 band into (i) the anterior smrface of the body of the hyoid bone 

 at its outer part, and (2) the adjacent portion of the great comu. 



Nerve-supply. — ^The posterior belly is supphed by the digastric 

 branch of the facial nerve ; and the anterior belly derives its nerve- 

 supply from the mylo-hyoid branch of the inferior dental, which is 

 a branch of the inferior division of the fifth cranial nerve. 



The posterior belly is directed downwards and forwards, and the 

 anterior belly downwards and shghtly outwards. 



Action. — (i) To elevate the hyoid bone, as in the act of deglutition ; 

 and (2) to depress the inferior maxilla. 



Relations. — The posterior belly is at first deeply placed, being 

 overhimg by the mastoid process, and lying under cover of the 

 trachelo-mastoid, splenius capitis, • and stemo-cleido-mastoid 

 muscles. It is also overlapped by the lower part of the parotid 

 gland. It crosses the external and internal carotid arteries, internal 

 jugular vein, and hjrpoglossal nerve. The stylo-hyoid muscle lies 

 above it, and the hypoglossal nerve below it for a short distance. 

 The intervening tendon is embraced by the fibres of the stylo-hyoid 

 muscle. This tendon crosses the hypoglossal nerve, and forms two 

 sides of a triangle, known as the triangle of Lesser, the base of which 

 is directed upwards, and is formed by the hypoglossal nerve. In 

 the area of this triangle is a portion of the hyo-glossus muscle, and 

 beneath this part is a portion of the Ungual artery. The anterior 

 belly, which is shorter than the posterior, is covered by the integu- 



