ANATOMY OF THE FROG 49 



open behind to the cavum aorticum, and anteriorly it is 

 continued into the carotid arteries and the right systemic 

 aorta. We have already noticed that the truncus arteriosus 

 bifurcates at its anterior end, and that each bifurcation very 

 shortly divides into three vessels. The bifurcations them- 

 selves are triple in constitution, as may be seen by cutting 

 them across, when each is observed to be made up of three 

 vessels an inner and ventral, the carotid; a median, the 

 systemic; and a dorsal and outer, the pulmo-cutaneous. It 

 is by means of these vessels that blood is conveyed from 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 inward and forward 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 backward branch which joins the systemic or 

 second arch, and is known as the ductus BotallL This duct, 



