PHYSIOLOGY. 939 
thing better than a hypothesis concerning their 
structure, as anatomists, even lately, disputed, whe- 
ther by far a larger organ in the human body, the 
uterus, possessed any muscular fibres or not. 
§ 234. 
Reducent vessels, (Vasa reducentia, s. medullaria), 
are of great number, and by far softer and more 
minute than the first. ‘They lie in the cellular tex- 
ture and in the pith, and run either m an oblique 
or horizontal direction. In their functions they re- 
semble veins. It is with great difficulty they can be 
filled with coloured liquors, and soon escape the 
eye of the observer. In some species of wood they 
become visible in their indurated state by a hori- 
zontal section. — 
$2355 
Air vessels, (Vasa pneumato-chymifera, vasa spi- 
ralia, fissure spirales, vel tracheae), are delicate, 
membranous, spiral and hollow tubes, which have 
other minute vessels, twisting round them in a spiral 
direction, like a cork-skrew, some close to each 
other, some more or less distant, fig. 282. The 
hollow interstices between them contain air, but no 
fluid, the spiral vessels themselves however contain 
a fluid. ‘The very thin membrane investing the hol- 
low interstice, occurs only in the more distantly 
twisted vessels; in those which are close to each 
other, though present it can scarcely be observed. 
They are commonly round; sometimes, however, 
by the circumambient presence of the other ves- 
cela 
sels, 
