4 1 6 ELEMENTAR Y ANA TO MY. [LESS. 



posteriorly into a relatively large middle sacral artery, and 

 gives off two considerable superior vesical (here called hypo- 

 gastric) arteries, which go to the umbilicus and much exceed 

 in size the primitive external iliac arteries. 



7. The significance of these changes will appear from a 

 consideration of the general condition of the arteries in the 

 Vertebrate sub-kingdom. 



A simple vesicular heart (e.g. in Amphioxus] may be con- 

 tinued on forwards (i.e. pre-axially) into a median artery, 

 whence on each side diverge very many pairs of arteries, 

 which ascend dorsally in contiguity with semi-cartilaginous 

 arches, subservient to aquatic respiration. In this case the 

 first arch on each side may continue uninterruptedly upwards 

 till it meets its fellow, the two uniting above to form the 

 anterior end of a long subaxial dorsal aorta. The ascending 

 arches behind the first do not reach the aorta, but taper as 

 they ascend till they terminate dorsally in a point, while each 

 has a contractile dilatation at its base. Meanwhile the 

 blood is collected by separate vessels (called veins) which 

 spring from each branchial arch, and, growing larger as 

 they ascend, pour their contents into the aorta. 



This is a condition found in the Lancelet alone amongst 

 the Vertebrata. In no other member of that sub-kingdom 

 can aortic arches by any calculation, or at any period of life, 

 be made to exceed eleven on each side. 



In the Shark Heptanchus there are probably seven distinct 

 branchial arches on each side. 



From the Lepidosiren and Ceratodus we find there may be 

 five branchial arches on each side ; or there may be but four, 

 as in the Perch. 



In Fishes the arteries run on the outer side of the branchial 

 arches (noticed in the Lesson on the Skull), and give off 

 minute twigs to membranous structures termed gills (which 

 will be described in the Twelfth Lesson). The blood is col- 

 lected in the gills and conveyed to corresponding ascending 

 vessels (called branchial veins) by minute twigs, 1 from which 

 such ascending vessels take origin. The branchial arteries 

 springing from the aortic bulb are not directly continuous, or 

 connected by considerable branches, with the vessels joining 

 the dorsal aorta or branchial veins. 



We may find, however, as in the Frog (at that age of its 

 tadpole condition when the external gills begin to atrophy) 

 that three branchial arteries from the bulb may co-exist 



1 See page 479, Fig. 405. 



