DISSECTION OF THE HEAD AND NECK. 151 



veins, the veins of the portal system, the hepatic veins, the anterior and 

 posterior vense cavee, and the veins of the brain. 



The Trachea, or wind-pipe (Plate 28), begins beneath the altanto-axial 

 articulation, where it is continuous with the larynx. It descends in the 

 middle plane of the neck, beneath the spinal column ; and, passing 

 between the first two ribs, it gains the thorax, where it bifurcates to 

 form the bronchi. In the neck the muscles of the region envelop the 

 trachea, and are related to it as follows : — The longus colli is related to 

 its upper aspect, the sterno-thyro-hyoideus extends along its lower face, 

 the sterno-maxillaris crosses its direction obliquely upwards and forwards, 

 the subscapulo-hyoideus crosses it obliquely downwards and forwards, 

 and at the lower part of the neck it contacts on each side with the 

 scalenus. It is also related to the oesophagus, the carotid artery, the 

 jugular vein, and the pneumogastric, sympathetic, and recurrent nerves. 



The Thyroid Body or Gland (Plate 29) is related to the upper part 

 of the trachea on each side, resting on its first four rings. The gland 

 has a rounded form, and a reddish-brown colour ; and it is richly 

 supplied with blood, which it receives from the thyroid and thyro- 

 laryngeal branches of the carotid artery. Sometimes a narrow isthmus 

 connects the right and left glands across the lower face of the trachea. 

 The gland has an investing capsule of fibrous connective-tissue, con- 

 tinuous with an internal trabecular framework. Under the microscope 

 the substance of the organ is seen to contain numerous spherical 

 spaces, each lined by a single layer of epithelium, and filled by a viscid 

 colloid material. 



The (Esophagus, or gullet (Plate 28), is a segment of the alimentary 

 canal. It begins above the larynx, where it is continuous with the 

 pharynx. It descends on the upper face of the trachea, and in the first 

 few inches of its course it lies in the middle plane of the neck, being 

 related to the longus colli muscle above. It soon, however, begins to 

 deviate to the left side, so that below the middle of the neck it lies 

 rather on the upper part of the left side of the wind-pipe. Maintaining 

 this relationship, the two tubes enter the thorax in company, the gullet 

 being prolonged through that cavity to pass by the foramen sinis- 

 trum of the diaphragm into the abdomen, where it terminates in the 

 stomach. The gullet is related to the muscles of the left side already 

 enumerated as contacting with the trachea, the sterno-thyro-hyoideus 

 excepted. It is also related to the carotid artery, the jugular vein, and the 

 pneumogastric, sympathetic, and recurrent nerves of the left side. The 

 cervical part of the oesophagus has the external appearance of a volun- 

 tary muscle, for which it is often mistaken at first sight by the student. 

 The examination of its structure, as well as that of the trachea, must 

 be postponed until the accompanying vessels and nerves have been 

 examined. 



