24 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, thinner walls, and darker colour, due to the 

 blood being seen through their walls. 



Dissect from the ventral surface. In cleaning a vein take hold 

 tcith the forceps^ not of the vein itself hut of the tissue surrounding 

 it ; and take especial care not to prick the vein^ as hy 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 2^cirts 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 connexion with the ventral surface of 

 the 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. 



3. Tlic 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 and 



