21 - carotid triangle

The carotid triangle is limited posteriorly by sternocleidomastoid, anteroinferiorly by the superior belly of omohyoid and superiorly by stylohyoid and the posterior belly of digastric. In the living (except the obese) the triangle is usually a small visible triangular depression, sometimes best seen with the head and cervical vertebral column slightly extended and the head contralaterally rotated. The carotid triangle is covered by the skin, superficial fascia, platysma and deep fascia containing branches of the facial and cutaneous cervical nerves. The hyoid bone forms its anterior angle and adjacent floor and can be located on simple inspection, verified by palpation. Parts of thyrohyoid, hyoglossus and inferior and middle pharyngeal constrictor muscles form its floor. The carotid triangle contains the upper part of the common carotid artery and its division into external and internal carotid arteries. Overlapped by the anterior margin of sternocleidomastoid, the external carotid artery is first anteromedial, then anterior to the internal carotid artery. Branches of the external carotid artery are encountered in the carotid triangle. Thus the superior thyroid artery runs anteroinferiorly, the lingual artery anteriorly with a characteristic upward loop, the facial artery anterosuperiorly, the occipital artery posterosuperiorly and the ascending pharyngeal artery medial to the internal carotid artery. Arterial pulsation greets the examining finger. The superior thyroid, lingual, facial, ascending pharyngeal and sometimes the occipital, veins, correspond to the branches of the external carotid artery, and all drain into the internal jugular vein. The hypoglossal nerve crosses the external and internal carotid arteries. It curves round the origin of the lower sternocleidomastoid branch of the occipital artery, and at this point the superior root of the ansa cervicalis leaves it to descend anteriorly in the carotid sheath. The internal laryngeal nerve and, below it, the external laryngeal nerve, lie medial to the external carotid artery below the hyoid bone. Many structures in this region, such as all or part of the internal jugular vein, associated deep cervical lymph nodes, and the vagus nerve, may be variably obscured by sternocleidomastoid, and, pedantically, are thus 'outside the triangle'.

20 - thoracic duct

The thoracic duct

This main lymphatic duct conveys all the lymph from the lower limbs, pelvis, abdomen, left side of the thorax and head and neck and left upper limb to the venous system; that is, all but the right superior quadrant of the body.

It drains the cisterna chyli, which is an elongated sac that lies on the right side of the bodies of the 1^st and 2^nd lumbar vertebrae below the diaphragm.

The thoracic duct passes superiorly from the cisterna chyli through the aortic hiatus in the diaphragm on the right side of the aorta. In the posterior mediastinum, the thoracic duct ascends among the thoracic aorta on its left, the azygos vein on its right, the esophagus anteriorly, and the vertebral bodies and the origins of the right posterior intercostal arteries posteriorly.

At about the level of the sternal angle the thoracic duct deviates to the left, posterior to the esophagus and ascends through the superior mediastinum into the root of the neck to empty into the venous system at the union of the left internal jugular and subclavian veins.

The duct has a number of valves, which prevent lymph flowing downwards and give it a beaded appearance.

The cysterna chyli receives lymph from the lower limbs, pelvis and abdomen. The thoracic duct receives the left bronchomediastinal trunk in the thorax, and in the neck the left jugular and left subclavian lymph trunks. All these trunks can open independently into the left internal jugular or subclavian vein.

Because the thoracic duct is thin walled and may be colorless, it may be difficult to identify during surgical procedures. If the thoracic duct is cut or torn during an accident, chyle (L. chylus, juice) escapes from it. This milky fluid contains a considerable amount of fine fat droplets. Leakage of chyle may be prevented by tying off the thoracic duct. The lymph then returns to the venous system by other lymph channels, which join the duct superior to the ligature. During digestion the thoracic duct is distended with chyle, a product of the small intestine.

A brachiocephalic venous catheterization is inadvisable on the left side since the insertion of a needle into the angle formed by the confluence of the internal jugular and subclavian veins might well encounter the terminal portion of the thoracic duct. The brachiocephalic route for a central venous catheterization on the left-hand side has been known to cause a chylothorax.