344 EPITHELIAL AND ENDOTHELIAL TISSUE. 



discernible, analogous to those of blood-vessels : the most inter- 

 nal is the endothelial, with an underlying delicate connective tis- 

 sue, which, as a rule, holds a number of elastic fibrillae; the 

 middle coat is composed of circular smooth muscles, inside of 

 which are sometimes found longitudinal fibers, and the outer 

 layer is fibrous connective tissue, with interspersed bundles of 

 smooth muscle-fibers. The reticulum of elastic fibrillae consti- 

 tutes a comparatively broad layer in the walls of the thoracic 

 duct j here, also, the oblique muscle bundles of the adventitial 

 coat are most distinctly marked. 



In the lower vertebrates the lymph-vessels are often found 

 dilated into lymph-sinuses, which, in part, are under the control 

 of muscles "lymph-hearts" (J. Miiller). Many of the lymph- 

 sinuses empty directly into larger veins. In mammals, such 

 lymph-sinuses are of doubtful occurrence, though it is generally 

 held that the lymph-follicles in the small intestine of rabbits are 

 surrounded by such sinuses. 



Von Recklinghausen discovered that the peritoneal cavity is 

 in direct communication with the lymphatic system at the ten- 

 dinous portion of the diaphragma. The peritoneum is expanded 

 over the large lymph- vessels of this portion, being lined by com- 

 paratively small endothelia, the cement-substance of which is 

 more or less closely perforated with minute openings, the stomata 

 and stigmata. The nuclei of the endothelia are arranged in the 

 form of a wreath around the stomata, and larger stomata are 

 again surrounded by a wreath of very small, regular endothelia. 

 Through these openings dissolved or finely granular colored 

 substances, if injected into the peritoneal cavity, are carried 

 directly into the lymphatics of the diaphragm (C. Ludwig, 

 Schweigger-Seidel). 



The method of Ludwig and Schweigger-Seidel ("Arbeiten aus der Physio- 

 logischen Anstalt zu Leipzig," 1867) is as follows: The abdominal cavity of 

 a rabbit which was bled to death is opened ; the V. cava, the aorta, and the 

 oesophagus are tied to the vertebral column by means of a common ligature, 

 and the corpse of the rabbit is divided into halves by an incision made below the 

 diaphragma. The upper half is suspended, the head downward, in such a 

 manner that the diaphragm assumes a goblet shape. A certain amount of 

 water, holding dissolved Prussian blue, is poured in this goblet, and by arti- 

 ficial respiratory movement the diaphragm is carried up and down for several 

 minutes. Next the peritoneal surface of the diaphragm is washed, and alco- 

 hol poured over it, in order to render the Prussian blue insoluble and fix the 

 tissues. After excision, the center of the peritoneal surface shows white ten- 

 dinous tracts bounding blue tracts. These latter are the larger lymphatics, 



