692 THE VEINS. 



The Sinuses of the Dura Mater m General. — These are vascular spaced 

 in the texture of the dura mater, or situated between that membrane and the 

 bones which form the walls of the cerebro-spinal sheath, or are even excavated on 

 the inner surface of these bones. These spaces differ more particularly from 

 the veins, by their being generally of a prismatic form, by being continually open, 

 by the absence of valves in their interior, and by the presence, in some of them, of 

 lamellas (Jrabeculce), or intersecting filaments [chorda WiUisii) which stud their 

 inner surface, and make them look reticulated. 



Their walls are reduced to an epithelial layer that lies either on the dura 

 mater, or on the osseous tissue. 



It is into these sinuses that the veins of the brain and spinal cord disgorge 

 themselves. 



The Sinuses of the Cranial Dura Mater in Particular. — Four prin- 

 cipal will be described : the superior longitudinal sinus of the falx cerebri, or 

 median sinus, the two cavernous or sphenoidal sinuses, and the group of occipito- 

 atloid sinuses. 



1. Sinus of the Palx Cerebri, Superior Longitudinal or Median 

 Sinus. — Channeled in the substance of the falx cerebri, and becoming wider 

 as it extends backwards, this sinus commences near the crista galli process, and 

 terminates on the internal parietal protuberance by bifurcating. The two 

 branches resulting from this division form the origin of the parieto-temporal 

 confluent, or winepress of Herophilus {torcular Herophili, Fig. 38(J, 1, 4). 



2. Cavernous or Supra-sphenoidal Sinuses. — These are two in number 

 — a right and a left. They occupy, on the internal face of the sphenoid bone, 

 at each side of the sella Turcica, the so-called cavernous fissures. Bordered 

 outwardly by the superior maxillary nerve, they receive at their anterior 

 extremity the insertion of the alveolar vein. Posteriorly, they join each other, 

 and in doing so form a kind of arch, open in front, around the pituitary gland 

 (Fig. 886, 8). Each opens widely at the foramen lacerum basis cranii, into the 

 subsphenoidal confluent. 



3. Occipital or Occipito-atloid Sinuses. — By this name is designated a 

 network of large irregular veins, situated beneath the external face of the dura 

 mater, on the sides of the occipital foramen, and on the entire internal surface 

 of the atloidean ring. Anteriorly, these venous reservoirs communicate, through 

 the condyloid foramen, with the posterior extremity of the subsphenoidal con- 

 fluent. Posteriorly, they are continuous with the spinal sinuses, of which we 

 may consider them to be the origin (Fig. 386, 6). 



4. Rudimentary Sinuses of the Cranial Dura Mater. — Independently 

 of the above-described reservoirs, there exists, on the inner wall of the cranium, 

 some rudimentary venous sinuses which should be indicated ; these are : 1. One 

 or two veins lodged in the structure of the tentorimn cerel^elli, designated the 

 petrosal or transverse sinuses, communicating, below, with the cavernous sinuses, 

 and entering, above, into the parieto-temporal confluent.^ 2. Some small, 

 irregular, and reticulated cavities, very variable in their disposition, situated 

 beneath the dura mater, on the sides of the cerebellar cavity, and which generally 

 empty themselves into the subsphenoidal confluent by traversing the posterior 

 part of the foramen lacerum basis cranii, and also opening sometimes into the 

 petrosal sinus ; these cavities may be regarded as the representatives of 



' More frequently, perhaps, tliese veias arise directly from the substance of the brain, and 

 do not communicate, below, with the cavernous sinuses. 



