484 LYMPHATICS. 



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, the left one terminating in the usual manner, the 

 right one opening into the right subclavian vein, in connection with the right 

 lymphatic duct. The thoracic duct has numerous halves 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 regurgitation 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 oeso- 

 phagus ; 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 coat 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 consists 

 of a layer of connective tissue, beneath which are several laminae of muscular 

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

 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. 248) 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 

 become considerably enlarged in cutaneous affections 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 its 

 substance ; 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 

 posterior 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 

 and posterior auricular lymphatic glands, and from thence join the cervical 

 glands. 



The superficial lymphatics of the face are more numerous than those of the 

 head. They commence over its entire surface, those from the frontal region accom- 

 panying the frontal vessels; they then pass obliquely across the face, accompanying 

 the facial vein, pass through the buccal glands on the surface of the Buccinator 

 muscle, and join the submaxillary lymphatic glands. The latter receive the 



