32 THE VASCULAR SYSTEM OF THE FROG 



4. The haemorrhoidal artery is a small median artery 

 arising from the hinder end of the aorta, and supply- 

 ing the large intestine. 

 c. Branches formed by the division of the aorta. 



1. The iliac arteries are the two large arteries formed by 

 'the division ot the aorta, and supplying the hind- 

 limbs. Each gives off a hypogastric artery, which 

 supplies the bladder, giving epigastric branches to 

 the ventral body -wall, and then continues as tlio 

 sciatic artery down the leg, giving off branches to 

 the muscles and skin of the thigh, and dividing at the 

 knee into peroneal and tibial arteries supplying the 

 leg and foot. 



HI. The Pulmo-cutaneous Arch is the hindmost of the three 

 aortic arches : it divides about the level of the carotid gland 

 into the following branches : 



1. The cutaneous artery is a large artery which at first 



runs forwards and upwards and then turns backwards, 

 supplying the skin of the back along the whole length 

 of the body, and sending smaller branches to the sides 

 of the head and to the skin of the ventral surface. 



2. The pulmonary artery runs with somewhat sinuous 



course along the outer side of the whole length of 

 the lung, giving off branches into its walls. 



D. The Structure of the Heart. 



Having completed the dissection of the bloodvessels, cut them 

 across, about half an inch from the heart; remove the heart com- 

 pletely, and dissect it carefully under water. It is well to cut the 

 vessels of unemjal lengths on the two sides, as this will facilitate 

 the recognitiUof the sides of the heart during the dissection. 



Place the heart at first with the dorsal surface upwards. 



1. The Sinus Venosus (Fig. 4, p. 25) is a thin-walled sac on 

 the dorsal surface of the heai't ; it is triangular in shape, with 

 the apex directed backwards. Into its anterior angles the right 

 and left anterior venae cavae open, and into its posterior angle 

 or apex the posterior vena cava. 



Cut away with scissors the dorsal wall of the sinus venosus so 

 as to expose its cavity : wash out any contained blood. 



