two stems. In the placental, the left duct of Cuvier (common 

 cardinal) is retained as the coronary sinus of the heart. 



Reptiles The head drainage pattern of the reptile is much 

 the same as that of the mammal (Figure 1 1-29). The inter- 

 nal jugular vein gives off a branch, the inferior jugular, to 

 the musculature of the throat; it then passes up behind and 

 over the tympanic cavity. Here it receives a posterior cere- 

 bral branch which passes through the vagus (jugular) fora- 

 men. This posterior cerebral stem is connected by a sigmoid 

 sinus with the middle cerebral vessel, which in part forms 

 the transverse sinus. The main lateral head vein passes for- 

 ward from the posterior cerebral stem, over the columella, 

 receives the middle cerebral vein emerging from the trigem- 

 inal notch, and goes forward into the orbit and the orbital 

 sinus. Between the orbital sinus and middle cerebral branch, 

 the lateral head veins are interconnected through the basis 

 cranii below the dorsum sellae. The orbital sinuses are also 

 interconnected through the interorbital septum. 



Branches from the orbital sinus serve the temporal region 

 and extend into the maxilla, passing forward through the 

 alveolar passage of that bone. There is also a pterygoid vein, 

 which in the roof of the mouth gives off a palatine division. 

 Anteriorly the orbital sinus connects through the orbitonasal 

 fissure with the nasal sinus; the nasal sinus is connected also 

 with the maxillary vein. 



The brain is largely drained by a dorsal sagittal sinus 

 which leads into transverse sinuses. The middle cerebral 

 stem may exit behind the trigeminal (Sphenodon) or in 

 front of (and over) that root (Lacerta and turtle). In some 

 lizards and in crocodiles, the posterior cerebral stem is re- 

 placed by another passing out through the foramen magnum. 

 The alligator lacks the middle cerebral connection. The 

 turtle has all of these stems. In the reptile a vena capitis 

 medialis is only slightly developed. 



In the case of the reptile, we see a primitive venous pat- 

 tern with internal vessels suggesting those which have be- 

 come the primary vessels of the mammal. 



anterior durai plexus 



middle plexus 



endolymphatic diverticulum 

 otic vesicle 



posterior dural plexus 



Gasserian ganglion 

 V 



sagittal plexus 



middle plexus 



transverse sinus 



posterior dural plexus 



sigmoid sinus 



ophthalmic vein 

 (root of anterior 

 plexus' 



internal jugular 



superior sagittal sinus 



sinus rectus 



occipital sinus 



transverse 



sinus 



sigmoid sinus 



inferior petrosal sinus 



Figure 1 1-33. Development of venous drainage of the head in the human. (After Streeter, 1918) 



CIRCULATION IN THE HEAD • 363 



