LYMPHATICS OF THE HEAD AND NECK. 389 



the vessels are small, and least numerously in the lower limbs where 

 the lymphatics are larger. In the thoracic duct an interspace of two 

 or three inches frequently occurs between the valves. Connected 

 with the presence of valves in the Ijonphatic vessels, are two lateral 

 dilatations or pouches, analogous to the valvular sinuses of veins. 

 These sinuses are situated on the cardiac side of the valves ; they re- 

 ceive the valves when the latter are thrown back by the current of the 

 blood ; and when reflux occurs, they become distended with a body of 

 fluid which makes pressure on the flaps. These pouch-like dilatations 

 and the constrictions corresponding with the line of attachment of the 

 convex borders of the flaps are the cause of the knotted appearance of 

 distended lymphatic vessels. 



Like arteries and veins, lymphatic vessels are composed of three 

 coats, external, middle and internal. The external coat is cellulo- 

 fibrous, like that of blood-vessels ; it is thin, but very strong, and 

 serves to connect the vessel to surrounding tissues, at the same time 

 that it forms a protective covering. The middle coat is thin and 

 elastic, and consists of a layer of longitudinal fibres analogous to those 

 of the innermost layer of the middle coat of arteries and veins. Some 

 few circular fibres may be seen externally to these in the larger lym- 

 phatic vessels. The internal coat is inelastic and more liable to rup- 

 ture than the other coats. It is a serous layer continuous with the 

 lining membrane of the veins, and invested by an epithelium. The 

 valves are composed of a very thin layer of fibrous tissue coated on its 

 two surfaces by epithelium. 



The lymphatic glands (conglobate, absorbent) are small oval and 

 somewhat flattened or rounded bodies, composed of a plexus of minute 

 lymphatic vessels, associated with a plexus of blood-vessels, and en- 

 closed in a thin cellular capsule. When examined on the surface they 

 are seen to have a lobulated appearance, while the face of a section is 

 cellular from the division of the numberless convolutions which are 

 formed by the lymphatic vessels within its substance. The colour of 

 the gland's is a pale pink, excepting those of the lungs, the bronchial 

 glands, which in the adult are more or less mottled with black, and 

 are sometimes filled with a black pigment. Lymphatic glands are 

 larger in the young subject than in the adult, and are smallest in old 

 age ; they as well as their vessels are supplied with arteries, veins and 

 nerves, like other structures. 



I shall describe the lymphatic vessels and glands according to the 

 arrangement adopted for the veins, commencing with those of the 

 head and neck, and proceeding next to those of the upper extremity, 

 lower extremity, and trunk. 



LYMPHATICS OP THE HEAD AND NECK. 



The Superficial lymphatic glands of the head and face are small, few 

 in number, and isolated; they are the occipital, which are situated 



