STRUCTURE OF THE CRANIUM. 157 



In the diploe of the dried bones, several arborescent channels* 

 may be seen by the removal of the external table. They were 

 discovered about the year 1805, by M. Fleury, while he was 

 Prosecteur at the School of Medicine in Paris: and engaged, at 

 the instigation of the venerable Chaussier, in some inquiries re- 

 lative to the structure of the cranium. The account given by 

 the latter is, that these channels are occupied in the recent sub- 

 ject, by veins, which, like all others, are intended to return the 

 blood to the heart. These veins are furnished with small 

 valves, have extremely thin and delicate parietes, and commence 

 by capillary ramifications, coming from the different points of 

 the vascular membrane, which lines the cells of the diploe. 

 Their roots are at first extremely fine and numerous, form by 

 their frequent anastomoses a kind of net-work, and produce by 

 their successive junction, ramuscules, branches, and large trunks, 

 which, becoming still more voluminous, are directed towards 

 the base of the cranium. Some variations exist in regard to the 

 number, size, and disposition of these trunks; but generally one 

 or two of them are found on either side of the frontal bone, two 

 in the parietal bone, and one on either side of the occipital bone. 

 Anastomoses exist between these several trunks> by which the 

 veins In the parietal bone are joined to those in the frontal and 

 in the occipital. Branches from the right side of the head also 

 anastomose with some from the left side. Besides the branches 

 already mentioned, one or two smaller than the others are di- 

 rected towards the top of the head, and terminate in the longi- 

 tudinal sinus. 



The descending veins of the diploe communicate in their pas- 

 sage with the contiguous superficial veins, and empty into them 

 the blood which they receive from the several points of the di- 

 ploe. These communications are passed through small fora- 

 mina, 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 the sinuses of the dura mater, and 

 partly into the venous plexus at the base of the pterygoid apo- 

 physes of the sphenoid bone, and form there the venous commu- 

 nications through the foramina of the base of the cranium, called 



* Chaussier, Exposition de la Structure de 1'Encephale. Paris, 1807. 



VOL. I. 14 



