52 COMPARATIVE ANATOMY 



form numerous loops of various sizes, and the capillary tubes 

 themselves unite freely with one another. Their walls are so 

 thin that they are hardly apparent, but the blood stream 

 coursing through their cavities is readily recognisable. The 

 capillary tubes vary in diameter from -jn^^th to T -Voth of an 

 inch. 



The capillaries are continuous, on the one hand, with the 

 ultimate subdivisions of the arteries. On the other hand, they 

 unite to form larger, but still very small vessels, the ultimate 

 subdivisions of the veins, also called venules. Both arterioles 

 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 





