LYMPHATIC VESSELS AND GLANDS. 



509 



III. THE LYMPHATIC VESSELS AND GLANDS. 



The lymphatic vessels correspond so closely in structure with 

 the small veins, as to require but a brief description. 



The finest (capillary) lymphatics have been seen with certainty 

 only in the small intestine, the mucous membrane of the trachea, 

 and the tail of the tadpole. They have a wall of simple membrane 

 without distinct nuclei, and a diameter of ^uu to T^OU f an inch. 

 (Fig. 339.) How these are changed into the larger lymphatic ves- 

 sels (Fig. 341, B and c), is unknown. Of the latter, the smallest 



Fig. 339. 



Capillary lymphatic from the tail of the tadpole, a. Simple membrane. 6. Process formed by it. 

 c. Remains of the contents of the cell forming these vessels, in which nuclei are concealed, d. Gra- 

 nules, forming lymph-corpuscles, e. Coecal termination of the vessels. /. One of the terminations, 

 still pretty distinctly recognizable as a formative cell. g. Isolated formative cells about to join 

 with the actual vessels. (KL'lliJcer.) 



are T j TO to y 5 of an inch, the larger 1 to 1 J lines. The last present 

 3 coats. The external coat contains longitudinal smooth muscular 

 fasciculi. The middle coat is composed of transverse muscular, 

 and fine elastic fibres. The tunica intima consists of a single, and 

 rarely double, elastic reticulated membrane, and an epithelium of 

 elongated, though still rather short, cells. (Fig. 142.) 



The thoracic duct has striped lamellas between the epithelium and 

 the elastic reticulated membrane, though the whole inner coat 

 scarcely measures 5 oVo to T^UO f an inch. The middle coat ( 7 | 

 of an inch thick) contains a transverse muscular layer and areolar 



