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HAND-BOOK OF PHYSIOLOGY. 



to assume the presence of any special channels at all, inasmuch as blood- 

 corpuscles can pass bodily, without much difficulty, through the walls of 

 the blood-capillaries and small veins (p. 159), and could pass with still 

 less trouble, probably, through the comparatively ill-defined walls of the 

 capillaries which contain lymph. 



FIG. 208. Lymphatic vessels of the head and neck and the upper part of the trunk (Mascagni). 

 1-6. The chest and pericardium have been opened on the left side, and the left mamma detached and 

 thrown outward over the left arm, so as to expose a great par t o f its deep surface. The principal 

 lymphatic vessels and glands are shown on the side of the head and face, and in the neck, axilla, and 

 mediastinum. Between the left internal jugular vein and the common carotid artery, the upper as- 

 cending part of the thoracic duct marked 1, and above this, and descending to 2, the arch and last 

 part of the duct. The termination of the upper lymphatics of the diaphragm in the mediastinal 

 glands, as well as the cardiac and the deep mammary lymphatics, is also shown: 



It is worthy of note that, in many animals, both arteries and veins, 

 especially the latter, are often found to be more or less completely eii- 

 sheathed in large lymphatic channels. In turtles, crocodiles, and many 

 other animals, the abdominal aorta is enclosed in a large lymphatic vessel. 



Stomata. In certain parts of the body openings exist by which 

 lymphatic capillaries directly communicate with parts hitherto supposed 

 to be closed cavities. If the peritoneal cavity be injected with milk, an 

 injection is obtained of the plexus of lymphatic vessels of the central 

 tendon of the diaphragm (Fig. 207); and on removing a small portion of 

 the central tendon, with its peritoneal surface uninjured, and examining 



