OF THE HEAD, FACE, AND NECK. 561 



seventh cervical vertebra, it curves downwards above the subclavian artery, 

 and in front of the Scalenus muscle, so as to form an arch ; and terminates near 

 the angle of junction of the left internal jugular and subclavian veins. The 

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

 goose-quill, 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 bifurcates, 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 con- 

 nection 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 towards the vein, so as to prevent the regurgita- 

 tion of venous blood into the duct. 



Branches. 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, 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 coal consists of a 

 layer of epithelium, resting upon some striped lamella, and an elastic fibrous 

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

 sists of a la} r er of connective tissue, beneath which are several laminae of mus- 

 cular tissue, the fibres of which are disposed transversely, and intermixed with 

 the 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 an inch in length, and a 

 line or a line and a half in diameter, which 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 the convex surface 

 of the liver, and terminates at the angle of union of the right subclavian and 

 right internal jugular veins. Its orifice is guarded by two semilunar valves, 

 which prevent the entrance of blood from the veins. 



LYMPHATICS OF THE HEAD, FACE, AND NECK. 



The superficial lymphatic glands of the head (Fig. 315) are of small size, few 

 in number, and confined to its posterior region. They are the occipital, placed 

 at the back of the head along the attachment of the Occipito-frontalis ; and the 

 posterior auricular, near the upper end of the Sterno-mastoid. These glands are 

 affected in cutaneous eruptions and other diseases of the scalp. In the face, the 

 superficial lymphatic glands are more numerous: they are the parotid, some of 

 which are superficial and others deeply placed in the substance of the parotid 

 gland ; the zygomatic, situated under the zygoma ; the buccal, on the surface of 

 the Buccinator muscle ; and the submaxillary, the largest, beneath the body of 

 the lower jaw. 



The superficial lymphatics of the head are divided into an anterior and a pos- 

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

 temporal set 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 set follow the course of the occipital artery, descend to the occipital 

 36 



