55Q 



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 

 among the cancellated tissue of the bones. In those of the cranium 

 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, a, 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 

 55t centri,' e. Many veins within 

 the cranium are included in 

 spaces formed by the separa- 

 tion of the laminae of the 

 dura mater, and do not admit 

 of being dilated beyond a 

 certain size : these e 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,' 

 Hg. 429, 5, 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 

 lumbar vertebra. 



