96 Dr Allen Thomson on the Vascular System 



pharynx. At first, this vessel divides itself only into two 

 branches situated immediately behind the mouth, which, passing 

 round the pharynx, reunite with one another on its upper part, 

 below the vertebral column, to form the descending aorta. 

 Shortly afterwards, a farther subdivision of the aorta takes place, 

 by which four new vascular branches (//?) are formed on each 

 side of the pharynx, behind the one which appeared first, and 

 leading from the ascending to the descending' trunks of the aorta. 

 These vessels, the primitive branchial arteries, are at first nearly 

 of a uniform diameter, and do not divide into any branches in 

 their course round the pharynx. Each artery runs along one of 

 the branchial plates or hoops, (Fig. 2, I). 



As development proceeds, four transverse clefts, the com- 

 mencing branchial apertures, appear between the branchial 

 hoops on the lower and lateral parts of the pharynx (Figs. 2 

 and 3, o). The four posterior hoops become stronger, more car- 

 tilaginous, and project farther from the side of the oesophagus, 

 and, at thersanie time, little-leaflets or tubercles, the rudiments of 

 those whieh afterwards form the conjb-hke fringe of the gill, 

 begin to be formed on their external sides. > Each of these- leaf- 

 lets, soon after its first appearance, is furnished by the large 

 vessel of the hoop with a little artery and vein, probably formed 

 in the same manner as the vessels of the tail and gills of the 

 salamander already alluded to. The number and size of the 

 leaflets gradually augment, and, at the same time, each of the 

 vascular arches is farther subdivided, till at last, when the 

 structure of thelgill is perfected, instead of the single arterial 

 vessel, which at first winds' round each hoop, there are formed 

 a branchial artery and vein, "^ the capillary vessels of which 

 join by a multitude of minute ramifications on the surface of 

 each of the leaflets. It has thus been' shown by- Rathke; that 

 the branchial vein is originally a continuation of the trunk of 

 the branchial artery. AVhile those changes are taking place, 

 the anterior vascular, arch on each side, not contributmg, like 

 the posterior, to form the vesselsiof the gills, gives off an arterial 

 branch (Fig. 1, t) at its anterior and convex part, which rises 

 to the head, ; and corresponds with the carotid artery of higher 

 animals. This vessel now no longer communicates with the 

 ascending aorta, but is supplied with blood by the posterior 



