Jeremiah S. Ferguson 67 
large central veins a few smooth muscle fibers are found, but these are 
always disposed in a longitudinal direction. As soon as longitudinal 
muscle fibers appear in appreciable numbers the venous wall acquires the 
type of the succeeding variety, the large central vein. 
In the large central veins, as in the small, but two coats can be readily 
distinguished. The inner coat, or intima, in these vessels consists of a 
lining endothelium, which rests upon a very thin membrane of delicate 
connective tissue, containing many elastic fibers. 
The outer coat, or adventitia, is also a very thin membrane of fibro- 
elastic tissue, but its fibrous bundles are coarser than those of the intima, 
and its elastic fibers form a very close network. The outer portion of 
this coat contains a few longitudinal smooth muscle fibers. The great 
majority of these fibers, however, are arranged in the form of longitudinal 
ridges which project into the adjacent medullary tissue. [’rom one to five 
of these muscular ridges occur in the circumference of the vein (Fig. 2 
and 3). Except at those points at which the muscle occurs, the venous 
wall is extremely thin (Fig. 3). The muscular ridges are frequently so 
large as to materially obstruct the lumen of the vessel (e. g., the middle 
vessel in Fig. 2, also the uppermost vessel, which is cut in very nearly 
longitudinal section), and they form so noticeable a peculiarity that 
their presence may be considered characteristic of this type of vessel. 
The writer has never failed to find these peculiar muscular ridges more or 
less highly developed in each of the human adrenals which he has ex- 
amined: he believes them to be constantly present. They are less highly 
developed in the suprarenal vessels of the lower mammals, but even there 
they may frequently be demonstrated. 
The muscle fibers of the larger ridges are arranged in bundles which 
are enveloped in fibro-elastic septa of connective tissue. All of the 
muscle fibers in these bundles are longitudinally disposed. This arrange- 
ment is well shown in Fig. 3, in which a large central vein is seen in 
transection at a point near the entrance of a large branch. Examination 
of sections somewhat higher in the series shows the union of these two 
vessels. 
In the section photographed, the branch has been longitudinally cut. 
The fine dark lines shown in the figure are bands of elastic fibers which 
are enveloped in delicate white fibrous tissue inclosing the cut ends of 
the bundles of smooth muscle. The tendency to form longitudinal ridges 
is shown in this figure by the irregular distribution of the muscle, one 
side of the vessel, in both the parent stem and the branch, beitg almost 
devoid of muscle fibers. The muscular character of these ridges is 
beyond doubt. 
