694 SPECIAL HISTOLOGY. 



The capillary lymphatics, which, in the three situations in which they 

 have hitherto been seen with certainty in the small intestine, the tail 

 of the Tadpole, and the mucous membrane of the trachea commence 

 partly with free prolongations, in part in networks. I have, in a single 

 instance in the tracheal mucous membrane in Man, had an opportunity 

 of investigation, and they were found to consist of a delicate structure- 

 less wall, without distinct nuclei after the addition of soda, and having a 

 diameter of 0-003-0-005-0-01 of a line (Fig. 235). The same structure is 

 presented in the simple lacteals of the intestinal villi in Mammalia, ex- 

 cept that these measure 0-012-0-026 of a line, and have a somewhat 

 thicker wall. The lymphatics discovered by me in the tail of the Tad- 

 pole (Fig. 288), on the contrary, correspond entirely with the blood-capil- 

 laries, in the occurrence of nuclei on the inner side of the very delicate, 

 structureless membrane, whilst they differ from them in being furnished 

 with short, jagged processes with prolongations. The diameter of the 

 lymphatic capillaries in this situation is 0-002-0-015 of a line, and the 

 two main trunks of the tail, like those of the blood-vascular system itself, 

 have a perfect capillary structure. 



In what way these capillary lymphatics are changed into the larger 

 lymphatic canals has not been seen by any one, or at all events has not 

 been investigated. The finest vessels, which have elsewhere come under 

 my observation, had a diameter of JQ-\-^ of a line, and these, except 

 in the thickness of the individual layers, corresponded in all respects 

 with the larger vessels of 1-1J lines. The latter, the medium-sized 

 lymphatics, present three tunics. The t. intima consists of an epithe- 

 lium, of elongated, although rather short cells, and of a single, rarely 

 double elastic reticulated membrane, longitudinally fibrillated, which, 

 as regards the thickness of its fibres, and the narrowness of the meshes, 

 exhibits manifold diversities ; but it never has thick fibres, nor does it 

 ever constitute a true elastic membrane (according to Weyrich, this 

 membrane is wanting in the lymphatics of the mesentery, whilst I have 

 always found it in those of the lumbar plexus, and in those of the ex- 

 tremities). This is succeeded by a stronger, t. media, composed of 

 transverse smooth muscles, with fine, also transverse elastic fibres ; and 

 lastly, there is a t. adventitia, with longitudinal smooth-muscular fasci- 

 culi. The latter I have found, in the extremities, upon vessels of not 

 more than l-10th of a line, and I consider them a good distinctive cha- 

 racter between lymphatics and small veins (vid. " Mikrosk. Anat.," II. 

 p. 236). 



The thoracic duct differs in some respects from the smaller lympha- 

 tics. The similarly constituted epithelium is succeeded by some striped 

 lamellcc, and these by an elastic reticular membrane, longitudinally 

 fibrillated, although the entire t. intima scarcely measures 0-006-0-01 

 of a line. The t. media, 0-025 of a line thick, commences with an 



