OF THE HEAD, FACE, AND NECK. 625 



subclavian vein at its angle of junction with the left internal jugular vein. The 

 thoracic duct, at its commencement, is about equal in size to the diameter of a 

 goosequill, diminishes considerably in its calibre in the middle of the thorax, and 

 is again dilated just before its termination. It is generally flexuous in its course, 

 and constricted at intervals so as to present a varicose appearance. The thoracic 

 duct not unfrequently divides in the middle of its course into two branches of 

 unequal size, which soon reunite, or into several branches, which form a plexiform 

 interlacement. It occasionally divides, at its upper part, into two branches, of 

 which the one on the left side terminates in the usual manner, while that on the 

 right opens into the right subclavian vein, in connection with the right lymphatic 

 duct. The thoracic duct has numerous valves throughout its whole course, but 

 they are more numerous in the upper than in the lower part : at its termination it 

 is provided with a pair of valves, the free borders of which are turned toward the 

 vein, so as to prevent the passage of venous blood into the duct. 



Tributaries. The thoracic duct, at its commencement, receives four or five 

 large trunks from the abdominal lymphatic glands, and also the trunk of the 

 lacteal vessels. Within the thorax it is joined by the lymphatic vessels from the 

 left half of the wall of the thoracic cavity, the lymphatics from the sternal and 

 intercostal glands, those of the left lung, left side of the heart, trachea, and 

 oesophagus; and, just before its termination, it receives the lymphatics of the left 

 side of the head and neck and left upper extremity. 



Structure. The thoracic duct is composed of three coats, which differ in some 

 respects from those of the lymphatic vessels. The internal coat consists of a single 

 layer of flattened lanceolate-shaped endothelial cells with serrated borders ; of a 

 subendothelial layer, similar to that found in the arteries ; and an elastic fibrous 

 coat, the fibres of which run in a longitudinal direction. The middle coat con- 

 sists of a longitudinal layer of white connective tissue with elastic fibres, external 

 to which are several laminae of muscular tissue, the fibres of which are for the 

 most part disposed transversely, but some are oblique or longitudinal and inter- 

 mixed with elastic fibres. The external coat is composed of areolar tissue, with 

 elastic fibres and isolated fasciculi of muscular fibres. 



The Right Lymphatic Duct is a short trunk, about half an inch in length and 

 a line or a line and a half in diameter. It terminates in the right subclavian vein 

 at its angle of junction with the right internal jugular vein. Its orifice is guarded 

 by two semilunar valves, which prevent the passage of venous blood into the duct. 



Tributaries. It receives the lymph from the right side of the head and neck, 

 the right upper extremity, the right side of the thorax, the right lung and right 

 side of the heart, and from part of the convex surface of the liver. 



LYMPHATICS OF THE HEAD, FACE, AND NECK. 



The Lymphatic Glands of the Head (Fig. 338) are arranged in the following 

 groups : (1) The occipital, one or two in number, placed at the back of the head, close 

 to the occipital artery. (2) The posterior auricular or mastoid, usually two in num- 

 ber, situated on the insertion of the Sterno-mastoid to the mastoid process. Both 

 these sets of glands are affected in cutaneous eruptions and other diseases of the scalp. 

 (3) The parotid or pre-auricular, some of which are superficial to, and others are 

 imbedded in, the substance of the parotid gland. (4) The buccal, one or more, 

 placed on the surface of the Buccinator muscle. (5) The internal maxillary, be- 

 neath the ram us of the jaw. (6) The lingual, two or three in number, lying on 

 the Hyo-glossus and Genio-hyo-glossus. (7) The retro-pharyngeal, lying one on 

 each side of the middle line in front of the Rectus capitis anticus major. 



The lymphatic vessels of the scalp are divided into an anterior and a posterior 

 set, which follow the course of the temporal and occipital vessels. The tem- 

 poral accompany the temporal artery in front of the ear, to the parotid 

 lymphatic glands, from which they proceed to the lymphatic glands of the 

 neck. The occipital follow the course of the occipital artery, descend to 

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