MINUTE ANATOMY OF THE LARGER LYMPHATICS. 299 



system of the blood capillaries, and the larger 'lymph vessels 

 which issue from the several organs, and ultimately unite to 

 form the main trunks. 



The larger lymphatics of Mammals and Birds are always 

 tubes, the walls of which agree with those of the bloodvessels in 

 their structure, and hence present a tunica intima very rich in 

 elastic fibres, and lined by a single layer of tesselated epithelium; 

 a tunica media, consisting exclusively of muscular elements; and 

 a tunica adventitia, composed as usual of loose connective tissue. 

 The tunica media does not attain the thickness of that in the 

 arteries, but its fibres pursue a similar transverse direction. 

 Upon the whole, the lymphatics are not so thick- walled as the 

 arteries, but, in the relation between the thickness of the wall 

 and the calibre of the vessel, assimilate much more closely to 

 the veins. The form of the lymphatics of Birds and Mammals 

 is peculiar, and so far differs from that of the bloodvessels, that 

 they are provided with very numerous valves, resembling gene- 

 rally the valves of the veins. Immediately above each valve the 

 vessel is somewhat wider than just below, and not unfrequently 

 there is a distinct saccular dilatation at this point. As a conse- 

 quence of this arrangement, the lymphatics only preserve their 

 cylindrical form for short distances in those parts which are 

 destitute of valves, whilst in those parts where the valves are 

 numerous they assume a varicose or moniliform appearance. 

 The valves, like those of the veins, are simply duplicatures of 

 the tunica intima. 



The structure and arrangement of the larger lymphatics 

 present essentially different features in the Amphibia. They 

 do not here form even approximative^ cylindrical tubes, but 

 lacunce, which occupy the interspaces between the several or- 

 gans. If, in consequence of an arrest of the flow of the lymph, 

 or by artificial injection, they become more completely filled 

 than is natural to them, they swell out in the form of large 

 sacs, which, however, possess no constant or definite form, since 

 they only represent interstitial spaces. As a general rule they 

 do not possess an independent thick wall, capable of being 

 detached from the surrounding parts, but their limits or boun- 

 daries are formed by the fascise and such condensed layers of 

 connective tissue as are found on the surface of the different 



