70 The Veins of the Adrenal 
gland, but occurs, more or less typically developed, in many of the large 
abdominal veins, notably in the renal veins and vena cava, into which the 
suprarenal veins empty. But nowhere is this peculiar venous type more 
strikingly developed, nowhere is the adventitia relatively so much thicker 
than the media, nowhere is a greater proportion of the smooth muscle of 
the venous wall longitudinally disposed, nowhere is there relatively less 
circular muscle, than in the suprarenal vein. Realizing the intimate 
relation of the parenchyma of the organ to its blood-vessels, and adopt- 
ing, if we may, the accepted physiological function of the adrenal—the 
formation of an internal secretion, a powerful vaso-constrictor which is 
poured into the blood within the capillaries and veins of the organ—the 
peculiar longitudinal arrangement of the muscular tissue, the valve-like 
protuberances at the junctions of the venous vessels, the absence of circular 
muscle from the walls of the veins of all sizes, and the general appearance 
of these vessels which are so remarkably different from the veins of most 
other organs, become, to say the least, extremely significant of a close 
structural relation, physiologically speaking, to the presence of an 
astringent secretion in the outflowing blood current. 
In this connection, one further observation is of importance. In the 
periadrenal connective tissue are numbers of small veins which return the 
abundant blood supply of the tissues of this region, most of them 
emptying into the phrenic veins. Many of these veins do not differ from 
the similar veins of other parts, but in many others the writer has ob- 
served that the muscle tissue is almost entirely disposed in a longitudinal 
direction, a condition which is quite the reverse of that found in the 
adipose and areolar tissues of other portions of the body. 
The writer also finds that many of the small veins of the adrenal, 
instead of opening into the central veins as is usually the case, pursue a 
less frequent course, penetrating the cortex and capsule of the organ, and 
emptying into the small veins of the surrounding connective tissue. The 
frequency with which this condition was associated with the occurrence 
of longitudinal muscle fibers in the periadrenal veins, suggests a more 
than casual relationship between the two conditions. 
SUMMARY. 
In conclusion, the above facts may be summarized as follows: 
1. The efferent blood-vessels of the adrenals form four successive vas- 
cular types, the sinusoids, the small central vein, the large central vein, 
and the suprarenal vein. 
2. Each of these types presents distinctive characteristics. 
