LYMPHATICS IN FROG LARVAE 15 



The neural plexus extends forward into the head, to lie 

 beneath the brain. It is fed as in the cord by the dorsal seg- 

 mental arteries as far forward as the diencephalon at the 

 forebrain flexure. 



The two ventral spinal arteries continue forward to this 

 point, joining posterior commissural branches of the carotids, 

 which give off cerebral arteries to form the plexus here and 

 forward under the remainder of the forebrain. A pathway 

 is thus open through the spinal and commissural arteries for 

 quick adjustment in the flow of blood from the cord to the 

 brain or the reverse, in case of excessive pressure in either 

 system. The neural plexus of the brain at this stage is found 

 to be different from that of the cord in sending penetrating 

 branches through the floor which continue dorsally in the side 

 walls of the vesicles to form plexuses over the roof. The 

 cerebral vesicles are then drained laterally by transverse veins 

 from these dorsal roof plexuses, and the veins from the nasal 

 and optic vesicles lying lateral to the brain join the vessels 

 from the forebrain plexuses to form two large veins, one 

 dorsal and one ventral to the region of the eye, which unite 

 with the ophthalmic in front of the auditory vesicle to con- 

 tinue back as the internal jugular vein. 



The neural plexuses of the mid-brain and hind-brain also 

 deliver most of their blood into the internal jugular vein, 

 through good-sized veins which form dorsally and posteriorly 

 to the auditory vesicle. But it is to be observed that, from 

 the first, some part of the outflow from the hind-brain plexus 

 takes an independent course into the first three intersegmental 

 veins. It must also be pointed out again, here, that the poste- 

 rior roof of the hind-brain carries no blood vessels. 



Just back of the ear the internal jugular vein continues 

 ventrally as the anterior cardinal vein (A.(\) into the pro- 

 nephric sinus at its anterior angle, and passing down along 

 its anterior wall, joins the posterior cardinal at the inferior 

 angle of the pronephros (figs. 1 and 2), to form the duct of 

 Cuvier. 



It is thus evident that only a small part of the blood from 

 the neural plexus in the head region passes out into the veins 



