CIRCULATORY SYSTEM OP TUB GRASS-3XAKE. 635 



anterior end of the kidney into the right efferent renal vein. On 

 the whole this distance appears to be greater in the female than 

 in the male. 



The remaining vessels of the male, the hepatic portal factors, 

 the anterior abdominal vein, and the epigastric vein correspond 

 in all respects to those of the female. 



Y. The Vessels of the Head. (Pis. LXXI. & LXXII.) 



(A) Arteries. 



It has been pointed out above that the whole of the blood 

 is brought to the head by the left common carotid, the right 

 common carotid having disappeared early in the course of 

 development. To compensate for this absence of an artery on 

 the right ride we find developed three arterial anastomoses 

 between the two sides of the head. The first lies beneath the 

 medulla oblongata and joins the two internal carotids ; the second 

 is situated beneath the fore-brain just in front of the optic 

 chiasma and unites the anterior cerebral and facial carotids ; and 

 the third is behind the symphysis of the lower jaw and joins the 

 two external carotids. 



The Left External Carotid (Carotis externa, Rathke; Arteria 

 inframaxillaris, Schlemm) arises from the common carotid internal 

 to the articulation of the lower jaw and the quadrate bone. It 

 runs forward between the floor of the pharynx and the broad 

 mylohyoideus * muscle, first inwards towards the tongue sheath 

 and then outwards to the inner side of the mandible, being 

 accompanied throughout the greater part of its course' by its 

 corresponding vein, the glossopharyngeal nerve, and the cutaneous 

 branch of the hypoglossal nerve. At the anterior end of the 

 lower jaw, just a short distance behind the symphysis, the left 

 external carotid anastomoses with its fellow by a well-marked 

 vessel. 



The Right External Carotid is similar to the left, save that the 

 common carotid from which it originated has disappeared and is 

 represented only by a small branch vessel It receives its blood- 

 supply partly from the anastomosis just mentioned, and partly 

 from the anastomosis between the internal carotids. 



The distribution of the arteries in the dorsal part of the head 

 is the same on both sides, so that the one description will apply 

 equally well to either side. 



The Internal Carotid (Carotis interna, Rathke ; Art. cephalica 

 and Art. carotis communis, Schlemm) starts from the origin of 

 the external carotid and bends in a sharp curve dorsally round 

 the angle of the lower jaw on the inner side of the vagus and 

 hypoglossal nerves. It then passes forward under the columella 

 and along the inner side of the quadrate to a point behind the 

 orbit and above the posterior pterygo-sphenoidalis muscle, where 



* The nomenclature of the muscles is that adopted hy Hoffiuann (23) 



42* 



