276 



for its strange and irregular form and for the different shapes of its arti- 

 cular surfaces and the great number of its projections, which render the 

 demonstration of this bone so complicated, and a knowledge of it so 

 difficult. 



It is more advantageous, withregard to the brain, that the skull should 

 be formed of several bones, than if it had consisted of a single bone. It 

 resists, more effectually, the blows it receives, their effect being lessened, 

 in passing from one bone to the other, and being interrupted, in the ob- 

 scure motions which they may experience at their sutures; its rounded 

 form increases, likewise, its power of resistance. This force would be 

 equal, in every point of the parietes of the cranium, if the form of that 

 cavity were completely spherical, and if the thickness of its parietes were, 

 in every part of it, the same. In that case, no fractures by contre coup 

 could occur, a kind of lesson occasioned by the unequal resistance of the 

 bones of the head, to the force applied to their surface. The pericrani- 

 um, the hairy scalp, the muscles which cover it, and the great quantity of 

 hair on its surface, serve, besides, to defend the brain, and are well cal- 

 culated to break the force of blows applied to the cranium. 



In addition to this hard and unyielding case, there lies over the brain, 

 a treble membranous covering, formed by the dura mater, which owes 

 its names to the erroneous opinion according to which it was supposed to 

 form all the other membranes of the body; it is further covered by the 

 tunica arachnoidea, so called from the extreme minuteness of its tissue, 

 and by the pia mater which adheres firmly to the substance of the brain. 



The dura mater lines, not only the inside of the skull and of the verte- 

 bral canal, which may be considered as a prolongation of it, but, like- 

 wise, penetrates between the different parts of the cerebral mass, sup- 

 ports them in the different positions of the head, and prevents mutual 

 compression. Thus, the greatest of its folds, the falx, stretched between 

 the crista galli of the ethmoid bone, and the inner protuberance of the 

 occipital bone, prevents the two hemispheres of the brain, between which 

 it lies, from compressing each other, when the body is on the side, and 

 maintains, on the other hand, the tentoriutn cerebelli in the state of ten- 

 sion necessary to enable it to support the weight of the posterior lobes of 

 the brain. This fold of dura mater is of a semi-circular form, separates the 

 portion of the skull which contains the brain, from that in which the cere- 

 bellum is situated. It is keot in a state of tension by the falx cerebri, on 

 which it also exerts the sjfce action : it does not present a horizontal 

 plane to the portion of brain which lies upon it, but one that slopes, in 

 every direction, towards the parietes of the skull, to which it transmits 

 most of the weight which it has to support. The tentorium cerebelli, 

 which thus divides the internal cavity of the skull into two parts of unequal 

 dimensions, is bony in some animals that move by bounding and with 

 rapid action; this is the case with the cat, which can, without being 

 stunned, take leaps from a considerable height. By means of this com- 

 plete partition, the two portions of the brain are prevented from passing 

 on each other, in the violent concussions which they experience. 



The tunica arachnoides, according to Bonn*, who was thoroughly ac- 

 .quainted with its structure, and who has given a very beautiful plate of 

 it, is the secretory organ of the serum which moistens the internal sur- 



Dissertatia de continuation} bits msmbranartim. Lug-dun. 4 Bat. 1763. 



