SCYLLIUM CANICULA 239 



in the frog, owing to the great shortening of the body region 

 that has taken place in the forms ancestral to Rana. 



Behind this, again, we find the small paired iliac or pelvic arteries 

 related to the pelvic girdle and fins. After this the dorsal aorta, 

 now considerably reduced in size, enters the haemal arches of the 

 caudal vertebrae and runs in them to the end of the tail. 



In addition to these more obvious vessels the dorsal aorta also 

 gives off a series of small paired parietal arteries, one pair to each 

 muscle segment of the body, and again in the region of the functional 

 part of the kidney a number of paired renal arteries. 



Before leaving the arterial system it will be as well to glance 

 briefly at the development of its most interesting part, namely, the 

 afferent and efferent arteries. In the early embryo with the gill 

 slits laid down, but before the gill filaments are developed, we find 

 coming off from the ventral aorta six pairs of vessels which pass as 

 uninterrupted arches, the aortic arches, around the pharynx in the 

 corresponding gill bars, the first one lying in the hyoid arch. On 

 the dorsal side of the pharynx the six arches on each side run into a 

 separate longitudinal trunk, so that there are at this time two dorsal 

 aortae running the length of the body which do not fuse to form a 

 single vessel until later. This is a very important stage from the 

 point of view of comparative anatomy, since it persists in the pharyn- 

 geal region of the adult Amphioxm and is also met with in a more or 

 less modified condition in the embryos of all the higher animals, 

 ourselves included. At a later stage the hyoid arterial arch degene- 

 rates, and with the development of the gill filaments the originally 

 continuous arches become split, so giving rise to the afferent and 

 efferent branchial arteries. 



Venous System. 



The most striking feature in the venous system is the 

 manner in which a number of the main vessels have become dilated. 

 They have lost their definite walls and spread out to form very irre- 

 gular spaces termed sinuses, whose relations to other veins are some- 

 times difficult to make out. For descriptive purposes it is convenient 

 to divide the veins into the anterior, those in front of the heart, 

 and the posterior, those behind. 



Anterior Veins. 



A very distinct channel runs from the snout back to the 

 heart on the dorsal side of the fish. This commences as a well- 

 marked crescent-shaped nasal sinus which lies on the postero-mesial 

 side of the olfactory organ. It communicates by means of a small 

 orbito-nasal vein, which passes through the cartilaginous hinder 



