26 THE VASCULAR SYSTEM OF THE FROG 



The veins should be dissected before the arteries, because, as 

 a. rule, they lie nearer the surface and are therefore met with 

 first. The veins are further distinguished from the arteries by 

 their larger size and darker colour, due to the blood being seen 

 more clearly through their thinner walls, and to the presence 

 of pigment on them; 



Dissect from the ventral surface. In cleaning a vein take 

 hold with the forceps, not of the vein itself but of the tissue sur- 

 rounding it ; and take especial care not to prick the vein, as by 

 doing so you allow the blood to escape and obscure the dissection, 

 and also render the vein itself difficult to see owing to the loss of 

 colour. Always dissect along and not across a bloodvessel, and 

 pin out the parts so as to stretch it slightly. 



I. Veins opening into the Sinus Venosus. 



a. The right anterior vena cava is a large vein opening 

 into the right side of the sinus venosus, and returning 

 to it the blood from the right side of the head and 

 body, and from the right fore limb. It is formed by 

 the union of three veins. 



1. The external jugular vein is formed by 



i. The lingual vein, from the floor of the mouth 



and the tongue. 

 ii. The mandibular vein, from the margin of the 



lower jaw. 

 In close connection with the ventral surface of 



each external jugular vein is a small round 



vascular body, the thyroid gland. 



2. The innominate vein is formed by 



i. The internal jugular vein, returning blood 

 from the interior of the skull, which it leaves 

 by an aperture at the posterior border of 

 the orbit. 



ii. The subscapular vein, a small vein from the 



back of the arm and shoulder. 



8. The subclavian vein, the largest of the three, 

 is formed by 



i. The brachial vein, from the fore limb. 



ii. The musculo- cutaneous vein: a very large 

 vein returning blood from the skin und 



