1040 



THE VASCULAE SYSTEM. 



^x Superior sagittal sinus 



Cerebral hemisphere 



Inferior sagittal sinus 



Internal cerebral vein 



7 Chorioid plexus of third ventricle 

 Tr^r=r^T.Vena basalis 



Vena basalis 



When the cerebral hemispheres increase in size the dura-matral tissue is compressed between 

 them, and between the cerebral hemispheres above and the mid- and hind -brain below, in the 

 form of folds (Figs. 836 A and B). As the folds are formed the conjoined anterior and middle 

 plexuses of one side are carried into relation with those of the opposite side in the median plane 

 of the head ; there the vessels of opposite sides unite together and are finally resolved into the 

 superior and inferior sagittal sinuses and the straight sinus (Figs. 837 A and B), and at the same 



time some of the 

 smaller vessels of the 

 plexuses which re- 

 tain their connection 

 with the piamater 

 are transformed into 

 the internal cerebral 

 veins and the great 

 cerebral vein ; and 

 from some of the 

 lower or ventral 

 Lateral ventricle tributaries, on each 



Chorioid plexus of lateral ventricle S } d *> } B . Produced 



the inferior cerebral 

 vein of the embryo 

 which probably be- 

 comes the vena 

 basalis of the adult 

 (Figs. 837 B, 838). 



Whilst the changes 

 last mentioned are 

 taking place the 

 growth of the hemi- 

 spheres forces the 

 upper part of the 

 middle stem tribu- 

 tary on each side 

 backwards and then 

 downwards until it 

 becomes the hori- 

 zontal part of the 

 transverse sinus (Fig. 

 838), whilst the an- 

 astomosis above the 

 otic region and the 

 posterior stem tribu- 

 tary are converted 

 into the sigmoid 

 portion of the trans- 

 verse sinus (Fig. 838). 

 By the time this 

 stage is attained the 

 anterior portion of 

 the primary head 

 vein which lies to 

 the medial side of 

 the semilunar gang- 

 lion has become the 

 cavernous sinus, and 

 the lower or ventral 

 part of the middle 

 stem tributary has been converted into the superior petrosal sinus (Fig. 838). 



The inferior petrosal sinus appears to be an independently formed anastomosis which 

 connects the posterior end of the cavernous sinus with the upper end of the internal jugular 

 vein across the medial side of the otic region (Fig. 838). 



The extra-cranial parts of the anterior cardinal veins become connected together, in the 

 upper or cephalic part of the thoracic region, by a transverse anastomosis which becomes the 

 greater part of the left innominate vein. A short distance cranialwards to this transverse 

 connexion, the primitive vein of the upper limb ultimately opens into the anterior cardinal 

 vein. The portion of the anterior cardinal vein of the right side, which lies between the 

 entrance of the limb vein and the transverse anastomosis, becomes the right innominate vein 

 (Fig. 839), and the corresponding part on the left side forms the commencement of the left 

 innominate vein. 



The part of the extra-cranial portion of each anterior cardinal vein which lies cephalwards 

 of the entrance of the limb vein forms the internal jugular vein ; and the part of the right 

 anterior cardinal vein which lies caudalwards of the transverse anastomosis becomes the upper 



FIG. 837 A. DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN SHOWING 

 SAGITTAL SINUSES STILL CONNECTED BY REMAINS OF THE PLEXUSES. 



Superior sagittal sinus 



- Cerebral 



Lateral ventricle 



Inferior sagittal 



sinus 



Great cerebral 



vein 



/ Vena basalis 



Mid-brain 



FIG. 837 B. DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN AFTER 

 COMPLETION OF THE SAGITTAL SINUSES. 



