THE LYMPH-VESSELS. 



107 



" - - N -- 



^^V% x 



Adventitia 



Lymph-cells 



FIG. 143. Transverse section of small lymph- 

 vessel. X 160. 



great variations in size. In some localities, as within the nervous tissues, they 

 surround even individual cells; in other places they are represented, in princi- 

 ple at least, by large cavities, since the subdural and subarachnoid spaces, the 

 chambers of the eye, the channels of the internal ear occupied by the peri- 

 lymph, the synovial sacs of the 

 joints, and, indeed, the great serous 

 cavities the pericardial, pleural and 

 peritoneal sacs are all regarded as 

 belonging to the lymph-spaces. 

 Although a conclusion not beyond 

 discussion, the lymph-sacs are now 

 believed to form a closed system of 

 intercommunicating channels, which, 

 while in intimate relation with the 

 lymphatic capillaries, do not actually 

 open into the latter. In many local- 

 ities, however, the spaces and capil- 

 laries are separated by only delicate 

 partitions which allow -the passage 

 of fluids, and also of wandering cells, 

 from the tissue-spaces into the 

 lymph-vessels. Within the adventi- 

 tious coat of certain blood-vessels, 

 conspicuously those of the retina, 

 the surrounding lymphatic channels constitute perivascular lymph-spaces. 

 The Lymph- Vessels. The definite lymph-paths include the capilla- 

 ries and the vessels. The lymphatic capillaries are arranged in networks, vary- 

 ing in closeness and complexity, and resemble in structure the blood-capil- 

 laries, consisting of a single layer of endothelial plates. They differ from 



the blood capillaries in being usu- 

 ally much greater in calibre and less 

 regular in size (30-60 //), larger 

 and smaller capillaries, often beset 

 with irregular constrictions and 

 enlargements, being indefinitely 

 interspersed. 



The more formal lymph-chan- 

 nels, the lymphatics, as they are 

 commonly called, which arise from 

 the networks of lymph-capillaries 

 and convey the lymph ultimately 

 to the subclavian veins, closely 

 resemble the veins in arrangement 

 and structure. The larger lymph- 

 vessels (from .5 mm. and upwards) 

 possess walls consisting of three 

 coats, which are much like those 

 of the veins. These include: (a) 

 the intima, composed of the endothelial lining and a thin layer of fibro- 

 elastic tissue; (6) the media, made up of circular involuntary muscle inter- 

 spersed with connective tissue and few elastic fibres; and (<:) the adventitia, 

 consisting of fibre-elastic tissue and, sometimes, of longitudinal bundles oi 

 smooth muscle. The numerous valves are essentially folds of the intima. 



Endothelium 



FIG. 144. Section of lymphatic, showing valve. X 180. 



