CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 509 



Valves are especially numerous, and in many of the vessels, as for 

 instance in those of the mesentery, just above each valve, where 

 the tube is somewhat swollen, the muscular fibres, which elsewhere 

 are chiefly disposed circularly, run in various directions so as to 

 form a contractile network. 



The smallest vessels, springing from the distinct lymph- 

 capillaries to be immediately described, consist of hardly more 

 than an epithelioid lining resting on a scanty connective-tissue 

 basis. The epithelioid cells are still fusiform and regular in shape, 

 and the calibre of each vessel is fairly uniform though, owing to the 

 valves which are exceedingly numerous, there is a great tendency 

 to become beaded. These smaller vessels like the others also 

 anastomose freely. 



Lymph- Capillaries. 



287. The smallest lymphatic vessels just described might, 

 from analogy with the blood vessels, almost be considered as 

 capillary vessels ; but the name lymph-capillaries is given to 

 vessels which joining and feeding those just described possess 

 very different characters. They are on the whole larger in calibre 

 than these, and distinctly larger than blood capillaries; they are 

 exceedingly irregular in shape, and in their junctions with each 

 other form irregular labyrinths rather than formal plexuses ; 

 they possess no valves and their only coat is an epithelium of a 

 very striking character. Like the blood capillaries their structure 

 is revealed by the action of silver nitrate. When a piece of tissue 

 containing lymph-capillaries, ex. gr. one taken from the tendinous 

 portion of the diaphragm, is examined after proper treatment with 

 silver nitrate, numerous spaces, on the whole tubular but highly 

 irregular in form, joining into an irregular labyrinth, are seen to 

 be lined with a layer of epithelioid plates of a peculiar kind. Each 

 plate or cell, which is more or less polygonal or at least not dis- 

 tinctly fusiform, is marked out by lines which are not straight and 

 even, but very markedly sinuous, the several bulgings of one cell 

 dove-tailing into the depressions of its neighbours and vice versa. 

 Such epithelioid plates of sinuous outline, or such a sinuous 

 epithelium, as we may for brevity's sake say, is characteristic 

 of the lymph-capillaries. A lymph-capillary is in fact merely 

 a space or areola of connective tissue, sometimes more or less 

 tubular but frequently irregular in form, lined by a single layer 

 of flat, transparent, nucleated epithelioid plates, each of which 

 possesses a remarkably sinuous outline. The lymph- capillaries 

 anastomose freely with each other and open into or join the 

 smallest regular lymphatic canals, which, many of them smaller 

 than the lymph-capillaries, are distinguished from these by their 

 more regular disposition, by their epithelioid plates being fusiform 

 with very little sinuosity of outline, and by the presence of valves. 



