170 THE ORGANS 



of mesotheliuni , the two being separated by a homogeneous basement 

 membrane. The stroma contains numerous lymphatics. These 

 have been described as communicating with the free surfaces by 

 means of openings — stoma tu. Recent observations, however, would 

 seem to indicate that these stomata are artefacts. 



That the lymph-vessels form a definite and closed system of 

 channels and are not in direct communication with the lymph spaces 

 has been clearly demonstrated. 



The origin of the lymphatic vessels is not clear. According to some investi- 

 gators they originate as evaginations from the vascular system, starting as four 

 buds, two from the veins of the neck, and two from the veins in the inguinal 

 region. The two anterior buds appear first (pig embryo of 14.5 mm.) and are in 

 communication with the anterior cardinal vein. A little later the posterior buds 

 connected with the posterior cardinals appear. From these buds the entire 

 lymphatic plexus develops by a process of evagination. 



According to other investigators, the lymphatic vessels first appear as min- 

 ute spaces in the mesenchyme of the axUla and groin (during second month in 

 human embryo). These spaces enlarge and by sending off branches give rise 

 to the network of lymphatic vessels. According to this veiw, the connection 

 between lymphatic vessels and veins is secondary. 



TECHNIC 



(i) Remove a portion of the central tendon of a rabbit's diaphragm. Rub 

 the pleural surface gently with the finger or with a brush to remove the mesothe- 

 lium. Rinse in distilled water and treat with silver nitrate as in technic 7, p. 82. 

 Mount in glycerin. If the silver impregnation is successful, the networks of 

 coarser and finer lymphatics can be seen as well as the outlines of the endothelium 

 of their walls. If care has been taken not to touch the peritoneal surface, the 

 peritoneal mesothelium and the stomata are frequently seen. 



(2) The Thoracic Duct. — Remove a portion of the thoracic duct, fix in forma- 

 lin-Miiller's fluid (technic 6, p. 7), and stain sections with hsematoxylin-eosin 

 (technic i, p. 20). 



General References for Further Study of the Circulatory System 



KoUiker: Handbuch der Gewebelehre des Menschen, vol. iii. 

 Stohr: Text-book of Histology. 



Schiifer: Histology and Microscopic Anatomy, in Quain's Elements of Anat- 

 omy, tenth edition. 



