THE DEVELOPMENT OP THE CRANIAL DURA MATER. 85 



fibrils has begun; isolated bundles may be made out everywhere in the lateral walls 

 of the sinus. This freedom from connective-tissue formation does not persist, 

 however, and the area is gradually invaded by the continued growth of the fibrils. 

 The avoidance of the region about the sinuses by the connective-tissue resolution 

 will be further commented on in the following subdivision of this paper. 



The dura, then, develops probably first in connection with the mesenchymal con- 

 densation which ultimately forms the bony skull and a portion of the dura (the cranial 

 periosteum). It first becomes apparent, as a structural unit, as a more cellular layer 

 differentiated, by a secondary condensation, out of the periaxial mesenchyme. As 

 the chondrogenous stage is approached it becomes differentiated as a distinct layer, 

 maintaining varying relationships with the inner perichondrium of certain of the 

 cranial bones. At a stage of 40 mm. in the fetal pig, the dura of the vertex may be 

 dissected out as a distinct, somewhat fibrous layer. The process of fibrous-tissue 

 transformation, however, is slow; the dura until late in fetal life shows an increased 

 number of nuclei, as does any young connective tissue. The invasion of the region 

 about the superior longitudinal sinus by connective-tissue fibrils is much more tardy 

 than is the transformation over the hemispheres. 



THE SUBDURAL SPACE AND THE MESOTHELIAL LINING OF THE DURA. 



The subdural space (cavum subdurale) has been the subject of controversy in 

 regard to its role in the pathway of the cerebro-spinal fluid. Before the work of 

 Key and Retzius^ 29 ) the view was held that the cerebro-spinal fluid occupied the sub- 

 arachnoid space in the spinal cord, but that in the cranium the subdural space 

 afforded an analogous pathway. This conception was largely due to the fact that, 

 in dissection on fresh material, the dura and arachnoid in the spinal region are found 

 to be in approximation; in the cranium the greater adhesion, by trabeculae, of the 

 arachnoidea to the pia renders the freeing of the dura from the leptomeninges the 

 simplest line of cleavage. This view was entirely disproved by the beautiful injec- 

 tions of Key and Retzius, who demonstrated the anatomical and physiological con- 

 tinuity of the subarachnoid spaces. 



With the introduction of this latter view by Key and Retzius the conception 

 of the subdural space naturally changed. These Swedish investigators demonstrated 

 a typical mesothelial cell-lining on the inner surface of the dura, as shown by the 

 method of silver reductions. Without an intimate connection with the true 

 cerebro-spinal fluid, the subdural space has come to be looked upon as somewhat 

 analogous to the serous cavities of the body. Quincke^ 46 ), after a subdural injection 

 of cinnabar granules, ascertained that communications existed between the subdural 

 and subarachnoid spaces, but only in the direction from subdural to subarachnoid. 

 Leonard HilK 24 ), from the results of physiological experiments, assumed that fluid 

 passed from the subdural to the subarachnoid space, and in the reverse direction, 

 with great ease. The more recent investigations, however, lend evidence to the 

 view that in the normal animal with undisturbed intracranial pressure relations the 

 two spaces are physiologically as well as anatomically separate. The current 



