THE HEART AND VASCULAR SYSTEM. 215 



which, however, they divide into afferent vessels, which, on emerg- 

 ing, unite into a smaller number of larger vessels, the efferent. 



In structure, they are composed of three coats the external 

 fibro-areolar, middle muscular, and internal or endothelial and 

 elastic. 



The lymphatic glands are generally situated in the course of 

 the blood-vessels, lymphatic vessels, or lacteal vessels, being ac- 

 cumulated together in certain localities, as the neck, abdomen, 

 axilla, etc. 



The lymphatic glands and vessels are named from the 

 regions they occupy or the vessels they accompany, and consist 

 usually of a superficial and deep set. Thus, we have cervical, 

 axillary, mediastinal, lumbar, inguinal, etc., and the lymphatic 

 vessels corresponding. 



The lacteals, or cliyliferous vessels, are the lymphatic vessels 

 of the small intestine, and differ only from the others in carrying 

 chyle during digestion from the intestines to the thoracic duct. 



THE THOKACIC DUCT is formed by the junction of the two 

 lumbar lymphatic trunks with the intestinal lymphatic trunk, in 

 front of the second lumbar vertebra, between the aorta and in- 

 ferior vena cava, as the receptaculum chyli, or cistern of Pecquet. 

 This receptacle is about one to two inches long and a quarter of 

 an inch wide. From its origin the thoracic duct ascends through 

 the abdomen, passes through the aortic orifice in the diaphragm, 

 and ascends behind the oesophagus between the aorta and azygos 

 vein to the fourth dorsal vertebra, where it passes to the left be- 

 neath the aorta, and ascends between the oesophagus and the left 

 subclavian artery to the last cervical vertebra, where it arches 

 forward, outward, and downward to enter the junction of the sub- 

 clavian and the left internal jugular vein at its posterior aspect. 

 Its orifice is protected by a pair of valves. It receives all the 

 lymphatic vessels below the diaphragm, those of the left side of 

 the head, neck, and left upper extremity. 



THE RIGHT LYMPHATIC DUCT is about half an inch long and 

 one-twelfth inch wide. It empties in a corresponding manner on 

 the right side to the thoracic duct. It receives all the lym- 

 phatics of the right side of the thorax, neck, head, and right 

 upper extremity. 



LYMPHATICS OF THE HEAD AND NECK. The substance of 

 the brain is probably destitute of lymphatics, but they are very 

 numerous in the pia mater and choroid plexuses of the lateral 

 ventricles, and pursue the same course as the principal veins, to 

 emerge at the base through the various foramina, to terminate in 

 the deep cervical glands. The occipital lymphatic vessels termi- 

 nate in the posterior auricular and occipital glands. The tern- 



