254 



TEXT-BOOK OF EMBRYOLOGY. 



porary, however, for collaterals arising from the veins pass to the lateral side of 

 the nerves and enlarge to form the main channels. The original channels atrophy 

 except in the region of the trigeminal nerves where they still remain on the 

 medial side of the nerves as the forerunners of the cavernous sinuses. The ves- 

 sel thus formed laterally to the cranial nerves (except the trigeminal) on each 

 side of the brain is known as the lateral vein of the head (vena lateralis capitis) 

 (Fig. 229). The blood is collected from the brain region by small vessels 

 which unite to form three main stems; one of these, the superior cerebral vein, 

 opens into the cranial end of the cavernous sinus; another, the middle cerebral 

 vein, opens into the opposite end of the cavernous sinus; and the third, the 

 inferior cerebral vein, opens into the lateral vein of the head behind the ear 

 vesicle (Figs. 229 and 224). The branches of the superior cerebral vein extend 



N.V N.VII N.IX 



Mid. cerebral vein- 



Sup, cerebral vein 



Inf. cerebral vein 



Lat. vein of head 



Hypoglossal 

 (XII) nerve 



Fig. 229. — Veins of the head of a 9 mm. human embryo. Mall. 



over the cerebral hemispheres and unite with their fellows of the opposite side to 

 form the superior sagittal sinus which lies in the medial line (Figs. 224 and 230). 

 The superior sagittal sinus is at first naturally drained by the superior cerebral 

 veins; but later, as the cerebral hemispheres enlarge and extend farther toward 

 the mid-brain region, it is carried back and joins the middle cerebral vein; still 

 later, for the same reason, it joins the inferior cerebral vein (Fig. 230, A and B). 

 During these later changes the connection between the superior sagittal sinus 

 and the superior cerebral vein is lost (Fig. 230). The middle cerebral vein 

 becomes the superior petrosal sinus which forms a communication between the 

 cavernous sinus and transverse sinus. The transverse sinus represents the 

 channel between the superior sagittal sinus and the cranial end of the cardinal 

 vein; or in other words, its cranial portion represents the connection between 

 the superior sagittal sinus and the inferior cerebral vein while its caudal portion 



