714 THE LYMPHATICS. 



finally, as the veins have three tunics, so have the lymphatics, these not differing 

 in any respect, except in being very much tliinner. 



If we pass into the domain of physiology, it is also easy to observe characters 

 which are common to the two anatomical systems under comparison. They, in 

 fact, almost equally divide the absorbent function between them — a function 

 which is accomplished in the radicular network of each ; and the dynamical 

 process which gives impulsion to the fluids they carry, if it is not quite identical 

 in both, is at any rate very similar in many points. 



We may, nevertheless, observe numerous differences between the veins and 

 the lymphatics, and chiefly in their form, number, capacity, and structure. 



The form of the lymphatic canals is, as we have said, nodulated and cyhn- 

 drical ; but their nodosities are much less marked, and are closer together than 

 in the veins, owing to the larger number and greater development of the valves. 

 Besides, as these vessels proceed for considerable distances, and preserve their 

 regularly cylindrical form with undiminished capacity, if we mentally bring all 

 the divisions of the lymphatic system to a single canal, we no longer have a 

 hollow cone the apex of which corresponds with the heart — although the capacity 

 of the lymphatic vessels augments from the trunk towards the branches ; for 

 this canal only represents a series of cylinders joined end to end, and successively 

 decreasing from its origin to its termination. 



The number of lymphatic vessels in a certain region is always much greater 

 than that of the veins. But as the lymphatics are much smaller than the veins, 



Fis. 391. 



A LYMPHATIC VESSEL WITH ITS VALVES. 



there is not, as might at firet be supposed, a proportional increase in their total 

 capacity. Observation, indeed, demonstrates that the relation between the 

 capacity of the lymphatics and the corresponding veins of a region does not 

 exceed one to two. 



The structure of the lymphatics differs from that of veins, in that there exists, 

 in those of average dimensions, smooth muscular fibres in the adventitious tunic. 

 The presence of muscular fibres in the external tunic of these vessels is rendered 

 necessary by the absence of an impelling organ at the origin of the lymphatic 

 system — such an organ being, in reality, disseminated throughout the extent of 

 the vessels, and aids the vis a tergo that causes the lymph to circulate in their 

 interior. 



"We terminate this short parallel, to notice in detail several points connected 

 with the general history of the lymphatics, and which merit particular attention ; 

 we allude to the origin, course, and termination of these vessels. 



Origin. — For a long period after the discovery of the lymphatic vessels, a 

 state of profound ignorance existed as to their origin. Nevertheless, the 

 importance of the solution of the problem was well appreciated, as it was really 

 the key to the theory of absorption ; numerous hypotheses, therefore, sprang 

 into existence. The anatomists who occupied themselves with the question, 

 were hindered in their investigation by the imperfect means of research at their 

 disposal. Beyond the larger branches, the lymphatics escaped attention, owing 



