180 THE FROG. 



lacunar spaces in the roof of the pharynx, which run together 

 to form a pair of continuous vessels. These are widest apart 

 opposite the first branchial arches (Fig. 77, A) ; further back, 

 behind the pharyngeal region, they lie close alongside each other, 

 and soon fuse to form the definite dorsal aorta. 



a. The arteries of the head are derived from the anterior ends 

 of the dorsal aortas. 



The carotid arteries (Fig. 78, AC), which are the direct con- 

 tinuations forwards of the dorsal aortae, become early connected 

 by two transverse vessels, the anterior and posterior commissural 

 vessels (Fig. 77, CA, CP), which, with the carotid arteries them- 

 selves, form an arterial circle surrounding the infundibulum. In 

 front of the anterior commissural vessel the carotid arteries are 

 continued forwards as the anterior cerebral arteries (Figs. 78 

 and 80, AR). 



The basilar arteries (Figs. 78 and 80, AB), appear shortly 



^before the mouth opens, as a pair of vessels which arise from the 



outer ends of the posterior commissural vessel, and run backwards 



along the ventral surface of the brain and spinal cord, not far 



from the median plane. 



The anterior palatine arteries (Figs. 78 and 80, AT) arise as 

 branches from the carotid arteries, just before these enter the 

 skull, and run forwards in the mucous membrane of the roof of 

 the mouth as far as the nose ; they appear very shortly after the 

 tadpole hatches. 



The pharyngeal artery (Fig. 80, AY) is formed, as already 

 described, from the dorsal part of the efferent vessel of the man- 

 dibular arch (Fig. 78, EM). From it the posterior palatine artery 

 {Fig. 80, AS) arises as a small, anteriorly directed branch, which 

 runs forwards and outwards in the roof of the pharynx. 



The lingual artery (Fig. 80, AL) is of considerable interest. 

 It appears shortly before the mouth opens, and in tadpoles of 

 about 9 mm. length is present on each side of the head as a very 

 short vessel, lying in the floor of the mouth immediately in front 

 of the truncus arteriosus. In the middle of its course it presents 

 a swollen bulb-like dilatation, from which arise : (i) an anterior 

 branch, the lingual artery proper, which runs forwards a short 

 distance, giving oif a small thyroid artery to the thyroid body ; 

 and (ii) a posterior branch, which runs a short distance outwards 

 and backwards towards the ventral end of the efferent branchial 



