174 MANUAL OF 1IISTOLOGY. 



In this respect it contrasts very decidedly with the blood-vas- 

 cular system. In the calibre of its vessels in different regions, 

 in different parts of the same organ, and even in different parts 

 of the same vessel, it is extremely irregular. A vessel of small 

 calibre may suddenly expand into a saccular shape, which 

 may have its diverticula or branches, or may form a chain of 

 lacunae. It is true that these dilatations are formed just in 

 front and behind the valves quite regularly, but they are also 

 found everywhere, being, in fact, a characteristic of these ves- 

 sels. 



Topographical peculiarities. Nor is a uniform direction to 

 be observed in the distribution of these vessels, for while they 

 usually accompany arteries, lying outside the accompanying 

 veins, they frequently take strange courses. A lymphatic may 

 suddenly leave its companions to strike across a comparatively 

 non-vascular field of tissue to share its fortunes with another 

 set of blood-vessels. Respecting the capillary lymphatics, 

 their place seems to be the middle ground between the blood- 

 capillaries, just where we would expect to find this drainage 

 system. 1 They lie deeper in the skin and mucous membrane 

 than the blood-vessels (Recklinghausen). The dispute con- 

 cerning the question as to whether the smaller lymphatics 

 have a distinct wall or are simple spaces, probably has been 

 largely due to the variation in the structure of the lymphatics 

 in the same tissue or organ in different animals, or in the same 

 animal at different ages. 



The thoracic duct, which represents the other extreme in 

 the structure of lymphatic passages, has an endothelial layer 

 supported by a reticulum of elastic fibres, which mingles with 

 the next layer, consisting of smooth muscular elements run- 

 ning in every direction, the transverse elements predominating. 

 The adventitia of connective- tissue fasciculi and elastic fibres 

 completes its coats. The muscular layer in man is highly de- 

 veloped compared with quadrupeds (Ranvier). 



1 On the external ear of a rat whose blood-vessels are injected with colored gela- 

 tine, and whose lymphatic vessels are rendered visible by silver, the larger centrifu- 

 gal lymph-vessels are seen, even with low powers, to be surrounded by a network of 

 blood-capillaries. The same has been demonstrated in the mesentery, the dia- 

 phragm, and the posterior extremities. Ueber ein die Lymphgefasse um-spinneudes 

 Netz von Blutcapillaren, von Alex. Dogiel. Arch. f. mikroskop.- Anat. Bd. 17. 3. 

 Heft, S. 335-340. 



