THE BRANCHIAL BLOOD-VESSELS. 171 



spaces along the roof of the pharynx, which by opening into 

 one another form a pair of longitudinal vessels ; these soon 

 extend backwards along the body, but remain for some time 

 distinct from each other. 



In the four branchial arches, blood-vessels are formed on a 

 definite plan. In the first and second branchial arches these 

 vessels appear immediately after the dorsal aortee, and are well 

 established at the time of hatching ; in the third and fourth bran- 



V.4 



EH- 



-VJ 



RT 



FIG. 79. A diagrammatic transverse section through the head of a 7 mm. 

 Tadpole, seen from behind ; the section is taken just behind the auditory 

 vesicles, and passes through the first branchial arch on each side. On the 

 right side both the afferent and efferent vessels of this arch are shown ; 

 on the left side the greater part of the afferent vessel has been removed 

 in order to expose the efferent vessel more thoroughly, x 50. 



A, aorta. AF.l, afferent vessel of first branchial arch. BH, medulla oblongata. 

 CH, notochonl. CP, pericardial cavity. EF.l, efferent vessel of first branchial arch. 

 G-, capillary loop of gill, connecting the afferent and efferent vessels together. Q,, sucker. 

 RT, truncus arteriosns. TP, pharynx. V, inferior jugular vein. VJ, anterior 

 cardinal vein. V.4, fourth ventricle. X, pneumogastric nerve. 



chial arches they arise in a similar manner, but at a somewhat 

 later stage. In the hyoid and mandibular arches, vessels com- 

 parable to those of the branchial arches appear at an early stage : 

 these, however, never quite conform to the type seen in the 

 branchial arches, and early undergo degenerative changes. 



a. The vessels of the first branchial arch may conveniently 



