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SINUSES. 



Sinuses of the Dura Mater. 



As these partitions arise like plaits from the internal surface of 

 the dura mater, there must necessarily be a cavity, larger or 

 smaller, between the external layer of the dura mater, which lines 

 the internal surface of the cranium, and the basis of the partition 

 or process which arises from it : this cavity must continue along the 

 whole basis of the partition, and a section of it will be triangular. 



Fig. 185.* 



This cavity is of considerable size at the upper edge of the 

 falx, where it rises from the dura mater, and also where it forms 

 the tentorium ; and at the posterior edges of the tentorium, 

 where it adheres to the occipital bone. 



The cavity at the upper edge of the falx is called the longi- 

 tudinal sinus ; that at the posterior edge of the tentorium forms 

 two cavities, called the lateral sinuses ; and that which is at the 

 junction of the falx and tentorium has the name of the torcular, 

 or press of Herophilus ; so named, from a supposition enter- 

 tained by the older anatomists, that the columns of blood, com- 

 ing in different directions, compressed each other at this point. 



* The sinuses of the upper and back part of the skull. 1. The superior 

 longitudinal sinus. 2, 2. The cerebral veins opening into the sinus from be- 

 hind forwards. 3. The falx cerebri. 4. The inferior longitudinal sinus. 5. 

 The straight or fourth sinus. 6. The venae G-aleni. 7. The torcular Herophili. 

 8. The two lateral sinuses, with the occipital sinuses between them. 9. The 

 termination of the inferior petrous sinus of one side. 10. The dilatations cor- 

 responding with the jugular fossae. 11. The internal jugular veins. 



