ANATOMY. 



casual obstruction to the ready transmis- 

 sion of blood through the lungs ; for the 

 whole of the veins are not distended in a 

 natural state, bvit serve as an occasional 

 reservoir, in which the blood, constantly 

 urged forwards by the heart, may be held 

 till the cause of obstruction has ceased. 

 Hut us such retardation in the course of 

 the venous blood would tend to drive 

 back the whole mass on the minute veins, 

 which art- tin- least able to bear it, such 

 r:i<!<- motion is prevented 1>\ valves, 

 which exist, in great numbers in the ve- 

 iioii-, system. These are thin membranes, 

 having- a semilunur 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, thev He close 

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

 tempts to return, the blood raises the 

 Igv. and that meets in the centre 

 of the vessel with the corresponding part 

 of the opposite valve, andthus closes tin- 

 fan..!. Thus, when an obstruction takes 

 place, each portion of a vein has to sup- 

 port that column of blood only which is 

 contained bet wren its own valves. Still, 

 as these vessels possess no powersof their 

 own, and are too far removed from the 

 heart to feel its influence on the passage 

 of blood through them, we find thai tne 

 circulation is affected in them by external 

 causes, as position, &<. Hence the legs 

 swell after long- standing; and hence also 

 ;s of thcsr parts are apttobecome 

 enlarged and varicose. 



TUxtrihiition nf thrvi-ins. Thisisfor the 

 most pait similar to thnt of the arteries, 

 as each of the latter vessclshave generally 



two accompanying veins, (\vhieh bear the 

 same names as the concomitant arteries) 

 named venae sodales arteriarum. I'ut in 

 some situations there is a class of veins not 



corresponding to the arteries, but running 

 under the skin, and termed cutaneous or 

 superficial veins. These are found par- 

 ticularly in the cx'reinities, and van-much 

 in si/e at different tin 



The vena cava superior is Conned by 

 three large trunks. 



1. Vena a/ygos, 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 tin; 

 axill. 



3. Left subclavian, formed in the same 

 manner as the rig-lit. 



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 tlic 

 blood from the brain. The dangvr in at- 

 tempts at suicide consists in dividing this 

 vessel or the carotid artery, and not the 

 external jugular vein. The :^ .I 1 >ry vein 

 is made up of the vesselswhich bring the 

 blood back from the arm. 1U sides the 

 veins, we have here a large 

 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. Hctwecn these in the 

 fore-arm are found some veins called the 

 ;!. At the bend '. these 



last, make up Uvo large trunks, of which 

 one opens into the basilic, and the other 

 into the cephalic vein. These are called 

 vena mediana basilica, and vena medians 

 cephulica. 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 

 is endangered by the lancet'. 



The inferior vena cava is a very large 

 trunk, running along the spine at the right 

 the aorta. It returns the blood 

 from all the lower parts of the body. It 

 is mack- 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 

 cavK hepaticx. 



The common iliac vein is formed by the 

 junction of the externa' and internal iliacs. 

 The latter brings back the blood from the 

 cavity of the pelvis ; the former returns 

 itfrom the lower extremity. 



\Vc have two large cutaneous veins to 

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

 phena major, which runs up along '.lie in- 

 ner side both of the leg and thigh, and 

 can be distinctly seen in the living 

 when in the ercc iphena 



minor, which runs over the calf of the leg- 

 The Cornier terminates in the femoral 

 vein near the abdomen, th_- la'tcr in the 

 popliteal vein. 



The vena portarum is a large \ 

 formed by the union of those veins which 

 belniig- to the stomach and intestines, th a 

 jrs the blood, 



\\ iiirh : -dtiiroiig-li ' 



to ihe liver, an i it l>r.me!i, s out in that 

 gland as arteries do in other par' 

 blood is returned fi\>m the liver by the. 

 hrp.iiic veins, which have been already 

 noticed. 



