56 OF THE CRANIUM. 



the diploe. These communications are produced through 

 small foramina which penetrate from the surface of the bone 

 to the diploe. The trunks of such diploic veins as are continued 

 to the base of the cranium, open partly into sinuses of the dura 

 mater, and partly into the venous plexus at the base of the 

 pterygoid apophyses, and form there the venous communications 

 called the emissaries of Santorini. Moreover, there are 

 communications sent from the diploic veins through the porosi- 

 ties of the internal table of the skull to the veins of the dura 

 mater. This fact is rendered very evident by tearing off the 

 skull cap, when the surface of the dura mater will be studded 

 with dots of blood, and the internal face of the bone also, 

 particularly in apoplectic subjects. It appears indeed that the 

 arteries of the cranium are principally distributed on its external 

 surface, and the veins on its internal surface and diploe. 



In the infant the diploic veins are small, straight, and have 

 but few branches : in the adult they correspond with the 

 description just given ; and in old age they are still more con- 

 siderable, forming nodes and seeming varicose. In children, 

 when the bones are diseased, they partake of the latter 

 character. In order to see them fully, the external table of the 

 skull must be removed with the chisel and mallet, both from its 

 vaults and base.]* 



The periosteum, which is on their externa\ surface, is called 

 pericranium. Internally the dura mater, or membrane which 

 covers the brain, supplies the place of periosteum. 



There are eight of these bones, which are thus denominated : 

 Os Frontis, Ossa Parietalia, Ossa Temjjorum, Os Occipitis, 

 Os Sphenoidcs, and Os Eihmoides. The two last are called 

 common bones, to denote that they are connected with the bones 

 of the face as well as with those of the cranium. 



The OS frontis forms the whole fore part of the vault of the 



* The diploe, or meditullium, corresponds exactly in structure and situation 

 ■with the spongy, or cellular tissue of the other bones of the body, though it has 

 unnecessarily received a distinct nanne. Neither are the diploic sinuses peculiar 

 to the bones of the skull. They are found presenting exactly the same appear- 

 ance in ilie bodies of the vertebra;, and appear in fact to be but a development 

 of the canaliculated tissue of the other bones. See Fig. 5, page 30. — p. 



