52 COMPARATIVE ANATOMY 



and venules are distinguished from capillaries by their walls, 

 which are thickened by the presence of muscular fibres and 

 connective tissue. The elastic layer, which is a conspicuous 

 element in the walls of larger arteries, dies out in the smallest 

 arterioles. 



The venules unite together and are gathered into larger and 

 larger vessels, the veins. Veins are vessels which convey 

 blood back to the heart. Their walls have much the same 

 structure as those of arteries, but there is much less elastic 

 tissue and much more connective tissue in them ; hence they 

 are less resilient, but at the same time tougher than arteries. 

 We may consider the veins under three heads : (i) those 

 bringing blood back to the heart from the head, throat, and 

 fore-limbs ; (2) those bringing back blood from the viscera, the 

 trunk, and hind limbs ; and (3) those bringing back blood from 

 the lungs. 



Blood is brought back from the tongue and from the lower 

 jaw by two veins, known as the lingual and mandibular veins 

 (/ and m fig. 9). These unite on the ventral wall of the 

 throat to form a single vessel, the external jugular vein, which, 

 after a short course, is joined by an internal jugular vein 

 bringing blood back from the inside of the skull. It leaves 

 the skull by an aperture at the hinder border of the orbit. 

 Just before it unites with the external jugular the internal 

 jugular receives a small subscapular vein bringing back blood 

 from the shoulder and back of the fore-limb. The short 

 section lying between the union of the subscapular with the 

 internal jugular and the external jugular is generally dis- 

 tinguished as the innominate vein. The single vessel formed 

 by the union -of the external jugular and innominate is almost 

 immediately joined by a third large vein, the subclavian. The 

 subclavian vein, which, as its name implies, lies under the 

 clavicle and precoracoid, is itself formed by the union of two 

 veins — the brachial which brings blood from the fore-limb, and 

 the musculo-cutaneous, a very large vein which takes its origin 

 in the nose, runs straight backwards as far as the middle of the 

 trunk, receiving branches from the skin and muscles of the 

 side of the body, and then turns sharply forwards to pass up 

 the ventro-lateral wall of the body and join the brachial. The 

 three veins, external jugular, innominate, and subclavian, unite 

 to form the superior vena cava of their side of the body, a 



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