1050 THE VASCULAR SYSTEM. 



The hepatic veins are parts of the primitive vitelline veins ; and the pulmonary veins are 

 splanchnic veins returning blood from a diverticula of the gut. 



It is noteworthy that some parts of the splanchnic venous system, i.e. the portal vein and the 

 coronary sinus, are portions of the most primitive vascular system, and that others, the 

 thyreoid, bronchial, mesenteric, vesical, and haemorrhoidal veins appear to belong to a somewhat 

 secondary group of splanchnic veins of combined segmental and intersegmental character ; more- 

 over, some of the secondary group of veins open into the primary splanchnic veins, e.g. the 

 superior and inferior mesenteric into the portal vein ; some open into the dorsal longitudinal 

 anastomosing veins, e.g. the vesical and haemorrhoidal veins open into the cardinal veins, which 

 are intersegmental anastomoses ; others again open into the internal jugular, which is part of 

 the anterior cardinal vein. 



Veins of the Limbs. The veins of the limbs, like the arteries, were probably at one time 

 intersegmental in character, but we have no indisputable proof that this was the case. Looked at 

 from an embryological standpoint, the most primitive limb veins are a superficial distal arch and 

 a post-axial trunk vein in each extremity ; at a later period digital veins are connected with the 

 distal arch, and a pre-axial trunk is formed. In the upper extremity the distal arch and its 

 tributaries remain as the dorsal venous arch and the digital veins, and the post-axial vein becomes 

 the basilic, axillary, and subclavian veins. The pre-axial vein of the upper extremity is 

 represented in the adult by the cephalic vein ; the latter vessel originally terminated in the 

 external jugular vein, above the clavicle, the union with the axillary portion of the post-axial 

 vessel being a secondary condition ; the primary condition is, however, frequently retained in 

 man, and is constant in many monkeys. The anastomosis between the pre-axial 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 small saphenous vein, which was originally continued proximally as the popliteal and 

 inferior glutaeal veins to the hypogastric portion of the posterior cardinal vein. 



The pre-axial vein of the lower limb becomes the great saphenous vein, which is continued 

 proximally to the cardinal portion of the left common iliac vein as the proximal part of the 

 femoral and the external iliac veins. 



The venae 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. 



ABNOKMALITIES AND 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 in ontogenetic develop- 

 ment, or to the acquirement of conditions which, though not as a rule present at any time in 

 man, occur normally in some 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, e.g. the trans- 

 ference of the subclavian artery to the front of the scalenus anterior, or of the ulnar artery to the 

 front of the superficial flexor muscles ; all of which, though undoubtedly due to alterations ot 

 ordinary developmental processes, still do not represent any known conditions met with, either 

 temporarily or permanently, in man or in other animals. Their occurrence cannot at present 

 be adequately explained, and their retention in the adult is entirely dependent upon their 

 utility. 



To the first and the 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 * 

 comprehensive knowledge of the development and morphology of the vascular system, to tl 

 chapters on which the reader is referred. 



