COMPARISON OF THE LYMPHATIC AND SEROUS SYSTEMS. 323 



view of the origin of the lymphatic vessels from serous canals. It 

 is not difficult, from these considerations, to obtain additional evidence 

 in favour of my theory ; nevertheless I do not venture to do so, since 

 we are treating of peculiar and, so to speak, embryonal tissues of 

 lymph capillaries that are, perhaps, as yet destitute of epithelium, 

 and in a very early stage of development ; connections and communi- 

 cations may therefore exist at this period, which at a later stage are 

 in some way or other modified or altogether abolished. 



If now we may consider the system of serous canals as the 

 origin of the lymphatic capillaries, the former system of tubes 

 appears to be adapted for the conduction of the proper fluids 

 of the tissues, whilst the latter constitutes the collecting tubes 

 which carry off the superfluous fluid. Regarded from this 

 point of view the comparison of the structural character of the 

 two systems is of great interest. Both are only sparingly pre- 

 sent in the denser tissues that are permeated by only moderate 

 quantities of nutritive fluid, as in the case of tendons; on the other 

 hand, in tissues like the central tendon of the diaphragm and 

 the mucous membrane of the intestine, in which the current of 

 the nutritious fluid of the tissue is extraordinarily rapid, the lym- 

 phatic vessels are very abundant and wide in relation to the 

 total sectional area of the serous canals ; lastly, the serous canal 

 system may have a great extension in relation to the entire 

 efferent system of the lymph path, in which case the tissues 

 are very soft and juicy, and the fluid in their interior undergoes 

 only slow interchange, and is, perhaps, on this account, especi- 

 ally adapted to the formation of new cells. To the last category 

 probably belong the looser masses of connective tissue which 

 invest the several organs, and unite the interstitial connective 

 tissue layers, on the one hand, with the serous and synovial 

 membranes on the other. In point of fact, the outer layers of 

 these tissues are very defective in continuity, whilst the serous 

 canals are extraordinarily wide ; the solid structures being only 

 present in the form of thin membranes and trabeculse, and we 

 know from pathological processes how quickly a cellular infil- 

 tration occurs in them. In the tunica adventitia of the blood- 

 vessels such cellular infiltrations have frequently been regarded 

 as lymphatic vessels in a state of distension. In certain parts 

 of the body this unusually wide serous canal system appears to 



