86 ELEMENTARY PHYSIOLOGY less. 



side of the head and neck, tlie right arm and right lung, 

 are connected by a common canal with the correspond- 

 ing vein of the right side. 



The lower part of the tlioracic duct is dilated, and is 

 called the receptacle of the chyle (Fig. 27, a.). This 

 part receives more particularly the lymphatics from the 

 intestines, which, though they differ in no essential re- 

 spect from other lymphatics, are called lacteals, because, 

 after a meal containing much fatty matter, they are filled 

 with a milky fluid termed chyle. Tlie lacteals, or lym- 

 phatics of the small intestine, not only form networks in 

 its walls, but send blind prolongations into the little pro- 

 cesses termed villi, with which the mucous membrane of 

 that intestine is beset. (See Lesson VL) 



Where the two principal trunks of tlie lymphatic system 

 open into the veins, valves are placed which allow of the 

 passage of fluid in one direction only, namely from the 

 lymphatic to the veins, the blood in the veins being 

 unable to get into the lymphatics, and in this way the 

 lymph from every part of the body is collected and re- 

 turned into the blood. 



2. The Origin and Structure of Lymphatics.— The 

 tissues of the body' are built up of cells wliich, though 

 lying closely applied to each other, ai-e often separated 

 by extremely minute spaces. These spaces are particularly 

 plentiful in that form of tissue of wliich we have already 

 spoken as connective tissue, as a result of its structural 

 arrangement, for it is made up of bundles of fine threads 

 or fibres which cross one another in all directions and thus 

 form a sort of feltwork of interlacing fibres (see Fig. 28). 

 Some of the spaces in this tissue are comparatively 

 large and are called areohe, whence this particular kind 

 of connective tissue is sometimes called areolar tissue 

 (see Lesson XII. i. This areolar tissue is, as we have said 

 (p. 13), present in every part of the body, and of course 

 supports tlie blood-capillaries, which are thus, in reality, 

 merely minute tubes lying imbedded in connective tissue. 



