CRANIAL DURA MATER 



913 



The cranial dura mater [dura mater encephali]. — The dm-a mater investing the 

 brain performs a double function — it serves as an internal periosteum for the 

 cranial bones and gives support and protection to the brain. In conformity with 

 its double function it consists of two layers, easily separable in the child, but 

 closely adhering to each other in the adult, except in occasional localities, where 

 there exist small clefts lined wnth endothelium. The large blood sinuses and ven- 

 ous lacunse, corresponding to the internal vertebral venous plexus of the vertebral 

 canal, are placed between the two layers and the semilunar gangha of the trigem- 

 ini also He between them. The cranial dura begins with the adhesion of the 

 spinal dura mater to the periosteum at the foramen magnum, and it forms a sac- 

 like envelope about the entire encephalon. Consisting of two layers, it is a much 

 thicker membrane than that of the spinal cord. 



The outer surface of the cranial dura mater when torn awa}-- from the cranial 

 bones appears very uneven, and when placed in water presents a flocculent 

 appearance. 



Fig. 719. — Coronal Section of the Head, Passing through the Posterior Horns of 



THE Lateral Ventricles. 

 From a mounted specimen in the Anatomical Department of Trinity College, Dublin. 



Dura matei 



Superior sagit- 

 tal sinus 



Falx cerebri 



Bulb of poste- 

 rior cornu 



Hippocampus 

 minor 



Inferior long- 

 itudinal fas- 

 ciculus 



Straight sinus 



Posterior 

 cornu of 

 lateral ven- 

 tricle 



Transverse sinus 



Dentate nucleus Tentorium cerebelli 



This is due to the many fine bundles of connective tissue and the blood-vessels which pass 

 between the dura and the cranial bones and which are partially pulled out of their openings in 

 the latter in the process of separation. The abundance of these connections, and, therefore, 

 the degree of adhesion to the bones, varies in different localities. The separation is much less 

 difficult from the inner table of the bones of the vault of the cranium than from the bones of 

 the base of the cavity. The adhesions to the vault of the cranium are most firm along the lines 

 of the sutures. This is due to the fact that during the period before the sutures are closed the 

 outer layer of the dura mater is directly continuous with the external periosteum, and, in 

 consequence of this condition during development, the connective-tissue connection is more 

 abundant along these lines and some is even caught in the closure of the sutures. Along the 

 vault there are occasionally noticed small lymph-spaces between the bone and the dura mater. 

 The stronger adherence to the base of the cranial cavity is- due to the numerous foramina in the 

 floor, through which all the larger cranial blood-vessels and the cranial nerves pass, and the 

 dura mater is continuous with the connective-tissue investments of these as well as with the 

 periosteum lining the foramina. Also the floor of the cavity is more uneven than the vault, and 

 the projections of the bones here tend to increase the firmness of attachment. The weight of 

 the brain upon the floor probably contributes to the result. 



The inner surface of the inner layer of the cranial dura mater forms the outer 

 boundary of the subdural cavity. Except for the occasional delicate subdural 

 trabeculae and the passage of blood-vessels and nerve-roots, this surface appears 



