553 



ANATOMY OF VERTEBRATES. 



428 



head and pectoral limbs, and intercostal spaces is poured by a 

 single precaval into the auricle. The lower intercostal veins of 

 the left side unite to form a hemi-azygos which joins the right or 

 main azygos vein. The plexiform disposition of the veins con- 

 tinues in those surrounding the myelon up to and including Man. 

 The diploic plexuses are the networks of veins which exist in and 

 amonc; the cancellated tissue of the bones. In those of the cranium 



o 



they form large irregular meshes of ampullated veins. These ves- 

 sels are very unequal in size, are subject to dilatations, and 



frequently end in culs-de-sac. 

 In the looser texture of the 

 vertebral centrums a vertical 

 section, as in fig. 428, ex- 

 poses the anterior, , and 

 posterior, b, portions of the 

 myelonal plexus ; the trans- 

 verse channel of anastomosis, 

 c, with the ( sinus centri,' d, 

 which bifurcates to unite 

 with the ( vena superficial 

 centri,' e. Many veins within 

 the cranium are included in 

 spaces formed by the separa- 

 tion of the laminse of the 

 dura mater, and do not admit 

 of being dilated beyond a 

 certain size : these ( sinuses ' 

 empty themselves, in Marsu- 

 pials and Ruminants and some 

 other quadrupeds, into tem- 

 poral veins, as well as into 

 the internal jugulars ; but in 

 Carnivora, Quadrumana, and 

 Man, almost wholly into the internal jugular vein. 



In all Mammals may be found the ' superior longitudinal sinus,' 

 fig. 429, s, uniting at t, the ' torcular Herophili ' with the lateral 

 sinuses, e. Besides these are the smaller * petrosal sinuses,' 

 superior and inferior, and the ( cavernous sinuses ' which are re- 

 servoirs of venous blood on each side of the sella turcica, crossed 

 by interlacing sclerous fibres. The cavernous sinuses communi- 

 cate with each other by the ( circular sinus,' and also with the 

 petrosal ones by the ( transverse sinus.' All the sinuses are 

 devoid of valves, and, by their freedom of intercommunication, 



Myelonal and diploic venous plexus and sinus : Human 

 lumbar vertebra. 



