CIRCULATORY SYSTEM OF THE GRASS-SNAKE. 



629 



lateral trunk, the lateral cephalic vein (Y. capitis lateralis, Grosser 

 and Brezina). At the same time the three cerebral veins become 

 connected by a median, longitudinal vessel (text-fig. 91). Further, 

 two new anastomoses arise from the median cerebral vein, one 

 goes to the anterior cerebral vein and the other, the secondary 

 median cerebral vein, leaving the skull with the trigeminus, goes 

 to the lateral cephalic vein. This becomes the main vein leading 

 from the anterior part of the brain. 



Text-fi^. 91. 



RC.V 



Diagram of veins in the region of the head in the embryo. It shows the original 

 veins indicated by the shaded portions and the definitive vessels indicated in 

 black. 



A.C.V. Anterior cerebral. A.V. Auditory vesicle. A.V.C. Anterior cardinal. 

 I.O.V. Infra-orbital. J.V. Right common jugular. L.C.V. Lateral cephalic. 

 M.C.V. Median cerebral. P.C.V. Posterior cerebral. P.V. Prosencephalic. 

 S.M.C. V. Secondary median cerebral. II, V, VII, IX, X, XII, Primordia of cranial 

 nerves. 



Adapted from Grosser and Brezina (19). 



The adult condition is reached by the regression of the dorsal 

 part of the anterior cerebral vein, the complete disappearance of 

 the portion of the anterior cardinal vein in the head, leaving its 

 cervical part, however, as the common jugular vein, and the 

 formation of a vein bringing back blood from the upper and 

 lower jaws. 



