628 THE L YMPHA TICS. 



that enormous gland placed on the track of the abdominal veins as the 

 glands are on parts of the lymphatics ? 



It may be added that, 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 

 cylindrical ; but their external nodosities are much less marked, and are closer 

 together than in the veins, owing to the larger number and greater develop- 

 ment of the valves. Besides, as these canals travel 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 obtain a hollow cone whose apex corresponds with the 

 heart, although the capacity of the lymphatic vessels augments from the 

 trunk towards the branches; this conduit 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, there is not, as might at first 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 adven- 

 titious 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 : this organ being, in reality, disseminated 

 throughout the extent of the canals, and aids the via a tergo that causes the 

 lymph to circulate in their interior. 



We terminate this short parallel, to dwell in detail on 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 to their transparency and tenuity. Thanks, how- 

 ever, to the patient and minute researches of Hunter, Cruikshank, Mascagni, 

 Fohmann, Panizza, Cruveilhier, and Sappey, the lymphatics were injected by 

 colouring matters or by mercury, and thus rendered visible to their finest 

 ramifications. 



It is now known that the lymphatics arise from capillaries, which form 

 networks or terminal culs-de-sac. 



