896 THE VASCULAR SYSTEM. 
and post-axial veins in the region of the elbow, and the connexion of the anastomosing 
channels, is brought about by newly-formed vessels of secondary character. 
The distal arch in the lower extremity and the tributaries connected with it remain in the 
adult as the dorsal venous arch of the foot and the digital veins. The post-axial vein becomes 
the external saphenous vein, which was originally continued upwards as the sciatic vein to the 
internal iliac portion of the cardinal vein; its connexion with the popliteal vein, and its more 
occasional connexion with the internal saphenous vein, being brought about by the formation of 
secondary anastomoses. 
The pre-axial vein of the lower limb becomes the long saphenous vein, which is continued 
upwards to the cardinal portion of the left common iliac vein as the upper part of the femoral 
and the external iliac veins. 
The ven comites of the arteries in both the upper and lower extremities are secondarily 
developed vessels which become connected with the upper portions of the pre-axial venous 
trunks. 
ABNORMALITIES OR VARIATIONS OF THE VASCULAR 
SYSTEM. 
Abnormalities are of special interest to the anatomist because of their morphological signifi- 
cance, and the vascular system is, perhaps more than any other, rich in such abnormalities, many 
of which are of great practical importance. ‘ 
With the exception of those irregularities which are directly due to the effect of morbid 
conditions and external influences, all abnormalities are the result of modifications of normal 
developmental processes. The exceptions referred to are, however, very numerous ; thus disease 
and external influences may lead to the obliteration of vessels, a condition which is invariably 
associated with the enlargement of collateral vessels, and it will be obvious that abnormalities 
so produced may occur in almost any situation. 
Abnormalities which are determined by, or are dependent upon, modifications of the usual 
developmental processes are of greater interest. In the human subject they are generally 
due either to the retention of conditions which normally are only transitory, or to the acquire- 
ment of conditions which, though not as a rule present at any time in man, occur normally in 
other animals. 
There are in addition other variations from the normal, such as the division of the axillary 
artery into radial and ulnar branches; the higher or lower division of the brachial artery ; the 
formation of “vasa aberrantia,” e.g. of long slender vessels connecting the axillary or brachial to # 
the radial, ulnar, or interosseous arteries; the altered position of certain vessels, as eg. the trans- 
ference of the subclavian artery to the front of the scalenus anticus, or of the ulnar artery to the — 
front of the superficial flexor muscles; all of which, though undoubtedly due to alterations of — 
ordinary developmental processes, still do not represent conditions met with, either temporarily — 
or permanently, in man or in other animals. Their occurrence cannot at present be adequately — 
explained, and their retention is entirely dependent upon their utility. 
To the first and last of these different groups of abnormalities it is not necessary to refer 
further, whilst with regard to the rest it will be sufficient to indicate those of greatest 
importance. They can only, however, be fully understood and explained on the basis of a 
comprehensive knowledge of the development and morphology of the vascular system, to the 
chapters on which the reader is referred. 
ABNORMALITIES OF THE HEART. 
The heart may be transposed from the left to the right side of the body, a condition which 
is usually associated with general transposition of the viscera, and with the presence of a right 
instead of a left aortic arch. 
The external form of the heart does not as a rule vary much, but occasionally the apex is 
slightly bifid, a character it normally possesses at an early stage of its development, and which is 
retained in the adult in many cetaceans and sirenians. The internal conformation of the heart 
deviates from the normal much more frequently ; more particularly is this the case with regard 
to the septa which separate the right from the left chambers. The interauricular septum may 
be entirely absent, as in fishes; it may be fenestrated and incomplete, as in some amphibians ; or 
the foramen ovale may remain patent, as in amphibians and reptiles. 
The interventricular septum may be absent, as in fishes and amphibians, or incomplete, as in 
reptiles ; when incomplete, it is usually the “ pars membranacea septi” which is deficient. 
ABNORMALITIES OF ARTERIES. 
The pulmonary artery and the aorta may arise by a common stem, as in fishes and some 
amphibians, and the common stem may spring either from the right or the left ventricle, or from 
both. In these cases the aortic bulb has remained undivided, and the normal position of the 
interventricular septum in relation to the lower orifice of the aortic bulb has been altered. 
