ANATOMY OF THE FROG 49 



the heart to all parts of the body. Vessels which carry blood 

 away from the heart are known as arteries. They have firm, 

 elastic walls, which do not collapse when empty, and retain 

 their circular section when cut across ; their firmness is due 

 to the relatively large amount of elastic tissue contained in 

 their walls. Veins are vessels which carry blood back to the 

 heart ; they have softer and less elastic walls than arteries, and 

 collapse when empty or when cut across. The arteries and 

 veins are united in the tissues by exceedingly fine vessels, 

 called capillaries, which can only be studied by the aid of the 

 microscope. The arterial system of the frog starts from the 

 truncus arteriosus, and the three main branches on either side 

 into which it divides. 



Of these three arches the carotids are the most anterior. 

 Each runs round the oesophagus towards the dorsal surface, 

 and, shortly after its origin, expands into a small almost spherical 

 dilatation the carotid gland. Just before it expands into the 

 carotid gland the arch gives off a small branch, which runs 

 inwards and forwards over the throat, nearly parallel to its 

 fellow of the opposite side, giving off branches to the hyoid 

 apparatus and the tongue. This is the lingual artery. The 

 carotid artery arises from the outer border of the carotid 

 gland, runs round the oesophagus towards its dorsal surface, and 

 there turns forward to run beneath the base of the skull, where 

 it divides into (i) the external carotid artery, whose branches 

 supply the pharynx, palate, and orbit, and (2) the internal 

 carotid, which passes through a foramen in the base of the 

 skull and supplies the brain. Just before it turns forward to 

 run under the base of the skull, the carotid gives off a small 

 branch backwards which joins the systemic or second arch, 

 and is known as the ductus Botalli. This duct, which is 

 open in the very young frog, usually becomes occluded in the 

 adult and no longer admits the passage of blood. The second 

 or systemic arch runs round the oesophagus on either side 

 towards the dorsal surface, and then turns inwards and back- 

 wards to join its fellow in the middle line just below the 

 vertebral column, at about the level of the sixth vertebra. The 

 right arch is directly continuous with the dorsal aorta, 

 which runs straight back beneath the vertebral column and 

 the urostyle; the left arch opens into the dorsal aorta by 

 a small opening, and is continued as the coeliaco-mesenteric 



