ANATOMY. 



casual obstruction to the ready transmis- 

 sion of biood through the lungs; for the 

 whole or* the veins are not distended in a 

 natural state, but serve as an occasional 

 reservoir, in whi-h the blood, constantly 

 urged forwards by the heart, may be held 

 till the cause of obstruction has ceased. 

 But as such retardation in the course of 

 the venous blood would tend to drive 

 back the whole mass on the minute veins, 

 which are the least able to bear it, such 

 retrograde motion is prevented by valves, 

 which exist in great numbers in the ve- 

 nous system. These are thin membranes, 

 having a seniilunar edge attached to the 

 side of the vein, and a straight edge float- 

 ing in the cavity of the vessel : they are 

 placed in pairs. When the blood is going 

 on in its natural direction, they lie close 

 to the sides of the tube ; but when it at- 

 tempts to return, the blood raises the 

 loose edge, and that meets in the centre 

 of the vessel with the corresponding part 

 of the opposite valve, and thus closes the 

 canal. Thus, when an obstruction takes 

 place, each portion of a vein has to sup- 

 port that column of blood only which is 

 contained between its own valves. Still, 

 as these vessels possess no powers of their 

 own, and are too far removed from the 

 heart to feel its influence on the passage 

 of blood through them, we find that the 

 circulation is affected in them by external 

 causes, as position, &c. Hence the legs 

 swell after long standing; and hence also 

 the veins of these parts are apt to become 

 enlarged and varicose. 



Distribution of the veins. This is for the 

 most part similar to that of the arteries, 

 as each of the latter vessels have generally 

 two accompanying veins, (which bear the 

 same names as the concomitant arteries) 

 named venae sodales arteriarum. But in 

 some situations there is a class of veins not 

 correspondingto the arteries, but running 

 under the skin, and termed cutaneous or 

 superficial veins. These are found par- 

 ticularly in the extremities, and vary much 

 in size at different times. 



The vena cava superior is formed by 

 three large trunks. 



1. Vena azygos, which returns the 

 blood from the sides of the chest, and runs 

 along the middle of the spine. 



2. Right subclavian, which is also made 

 up by three venous trunks, viz. the inter- 

 nal jugular, the external jugular, and the 

 axillary. 



3. Left subclavian, formed in the same 

 manner as the right. 



The external jugular vein returns the 

 blood from the outside of the head, and 

 runs along the neck, just under the skin. 



We sometimes bleed from this in affec- 

 tions of the head. 



The internal jugular is a very large ves- 

 sel, lying deeper in the neck, and close to 

 the carotid artery. It brings back the 

 blood from the brain. The danger in at- 

 tempts at suicide consists in dividing this 

 vessel, or the carotid artery, and not the 

 external jugular vein. The axillary vein 

 is made up of the vessels which bring the 

 blood back from the arm. Besides the 

 deep-seated veins, we have here a larg^ 

 superficial vessel, running along the out- 

 side of the fore-arm and arm, and called 

 the cephalic vein ; another on the inside, 

 named the basilic. Between these in the 

 fore-arm are found some veins called the 

 median. At the bend of the elbow these 

 last make up two large trunks, of which 

 one opens into the basilic, and the other 

 into the cephalic vein. These are called 

 vena mediana basilica, and vena mediana 

 cephalica. It is the latter veins that we 

 generally bleed, when that operation is 

 performed in the arm ; and as they run 

 directly over the artery, the latter vessel 

 is endangered by the lancet. 



The inferior vena cava is a very large 

 trunk, running along the spine at the right 

 side of the aorta. It returns the blood 

 from all the lower parts of the body. It 

 is made up by the junction of the two 

 common iliac veins and as it ascends 

 through the abdomen, it receives the fol- 

 lowing venous trunks ; the lumbar, sper- 

 matic, renal, and the immense venae 

 cavae hepaticae. 



The common iliac vein is formed by the 

 junction of the external and intei lal iliacs. 

 The latter brings back the blood from the 

 cavity of the pelvis; the former returns 

 it from the lower extremity. 



We have two large cutaneous veins to 

 notice in the leg and thigh : viz. the sa- 

 phena major, which runs up along the in- 

 ner side both of the leg and thigh, and 

 can be distinctly seen in the living person 

 when in the erect posture ; the saphena 

 minor, which runs over the calf of the leg. 

 The former terminates in the femoral 

 vein near the abdomen, the latter in the 

 popliteal vein. 



The vena portarum is a large vessel, 

 formed by the uniou ofthose veins which 

 belong to the stomach and intestines, the 

 spleen and pancreas. It conveys the blood, 

 which has circulated through those organs 

 to the liver, and it branches out in that 

 gland as arteries do in other parts. Its 

 blood is returned from the liver by the 

 hepatic veins, which have been already 

 noticed. 



