DISSECTION OP THE HEAD AND NECK. 147 



Each valve is composed of two or three seraihinar folds of the inner coat 

 of the vein, the folds having a close resemblance to the semilunar seg- 

 ments of the aortic valve (Fig. 11, page 129). Each flap with the adjacent 

 part of the wall of the vein forms a small pouch with its mouth directed 

 towards the heart. When the blood tends to regurgitate, it distends 

 these pouches until the segments meet across the vessel and thus arrest 

 the backward current. In most veins throughout the body similar 

 valves are found; but they are most numerous in the veins of the limbs. 

 In the small veins each valve may be composed of only a single flap. 

 The following veins have few or no valves : — the pulmonary system of 

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

 posterior vense cavse, 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 inthmns 

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

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

 with an internal trabecular framework. Under the microscope the sub- 

 stance 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, Avhere 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 



