THE VASCULAR SYSTEM. l6l 



THE LYMPHATIC SYSTEM. 



The lymphatics constitute that part of the vascular 

 system which collects the colorless fluid called lymph 

 from the tissues, and the chyle from the small intestine, 

 and conveys them to the external jugular veins. The 

 portion of the lymphatic system originating in the villi 

 of the small intestine and conveying the chyle to the 

 left thoracic duct is known as the lacteals. 



The lymphatic system consists of four parts: (i) the 

 lymphatic spaces between the skin and muscles in all 

 parts of the body, the spaces between the muscles adjacent 

 to the bones, and internal spaces such as those enclosed 

 by the peritoneum and pleura; (2) minute thin-walled 

 vessels beginning by capillary mouths in the spaces just 

 described and leading to larger vessels which empty into 

 the tracheal trunks or thoracic duct ; (3) the glands, which 

 vary in size from a mustard-seed to two centimeters in 

 diameter, and are located chiefly on the side of the neck, in 

 the axillary region, at the bifurcation of the trachea; in 

 the inguinal region, lumbar region, and mesentery; (4) 

 the two tracheal trunks and thoracic duct (Fig. 78). 



One of the largest of the lymphatic spaces is the 

 greater cavity of the peritoneum. The cavity of the 

 lesser peritoneum, a large lymphatic space, may be 

 shown by separating the layers of the great omentum 

 carefully. The cavity of the pleura within the chest is 

 also a large lymphatic space. Numerous small lymph 

 spaces are present in all parts of the body. The lym- 

 phatic capillaries originating in these spaces are invisible 

 to the naked eye, but their union into larger vessels may 

 be demonstrated satisfactorily in a cat just killed, by 

 pulling forward the tongue, and injecting very slowly 

 beneath the skin, on its ventral aspect, five or ten centi- 

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