410 STRUCTURE OF THE MUCOUS MEMBRANE. 
slightly flattened, the base of the villus having an oval form. 
Such a villus as the one now described, divested of epithe- 
lium, measured ^ of an inch in length, -fa of an inch in 
greatest breadth at the base, and from T ^3- to of an inch 
at the summit ; and this may be taken as the average mea- 
surement of the villi. Some are smaller, and more or less 
cylindrical in form ; one of these measured ¥ fa in diameter, 
and others are broader and flatter, and form a transition to 
the lamellated membrane. The length is the same in all. 
In structure, each villus is composed of the transparent 
granular tissue of the corium enclosed in limitary membrane ; 
a capillary plexus, with its afferent artery and efferent vein, 
a lacteal vessel, and probably a nervous loop. The capillary 
plexus occupies the surface of the villus lying immediately 
beneath the limitary membrane, while the artery and vein 
are situated in the interior substance of the villus, running 
sometimes together, and occupying the central axis or one 
side of the villus, but more frequently apart, in which case, 
and particularly in the flat villi, the artery ascends the villus 
on one side, while the vein descends on the other. In its 
course within the villus, the artery gives off branches, which 
form the capillary plexus with large open meshes of the sur- 
face of the villus. 
The ground of the mucous membrane between the bases of 
the villi is of small extent in comparison with the surface 
occupied by the villi themselves, the latter appearing to be as 
closely placed together as is possible for a number of oval- 
shaped discs, which represent the bases of the villi. This 
ground surface, which nowhere approaches in extent to the 
diameter of a villus, is concave, rising on all sides into the 
slopes of the villi. In structure, it is composed of a capillary 
plexus, with large open meshes. The capillary plexus is con- 
tinuous with that of the villi, and the large meshes are occu- 
pied by the openings of the crypts or simple follicles of the 
mucous membrane. In the centre of the area between the 
villi, there is a cluster of three, four, or five follicular openings; 
while around the base of each villus there is a more or less 
complete circle of such openings. 
It follows from this description, that if a villus were cut 
off near its base, the latter would be seen to be surrounded 
by a circle or zone of follicular apertures ; or if a villus re- 
mained undeveloped, the same appearance would be obvious. 
Now, this latter phenomenon does take place; certain of the 
villi are undeveloped, aborted villi ; they have the oval shape 
of the base of a villus, with the circle or zone of follicular 
openings around their circumference, the central area being 
