30 THE VASCULAR SYSTEM OF THE FROG. 



4. The haemorrhoidal artery is a small median artery 

 arising from the hinder end of the aorta, and supplying 

 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 of the aorta, and supplying the hind-limbs. 



Each gives off a hypogastric artery, which supplies 



the bladder, giving epigastric branches to the ventrtil 



body-wall, and then continues as the 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. 



III. 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 wdiich 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 substance. 



D. The Structure of the Heart. 



Having completed the dissection of the blood-vessels, cut them 

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

 pletely, and dissect it carefidly binder water. It is well to cut the 

 vessels of nnequcd lengths on the two sides, as this loill facilitate the 

 recognition of the sides of the heart during the dissection. 



Place tJie heart at first with the dorsal surface upwards. 



1. The Sinus Venosus (Fig. A, p. 23) is a thin-walled sac on 

 the dorsal surface of the heart ; it is triangular in shape, with 

 the apex directed backwards. Into its anterior angles the right 

 and left tyiterior vena? cavse open, and into its posterior angle 

 or apex the posterior vena cava. 



Cut away with scissors the dorsal ivall of the sinus venosus so as 

 to expose its cavity : ivash out any contained blood. 



