1 8 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. i. The skull with the three largest sensory nerves of the face and the middle meningeal artery. 



(i) By abnormally developed Pacchionian depressions, situated chiefly near the median line 

 of the cranial vault and due to marked proliferation of the villi of the arachnoid, that is, the 

 Pacchionian bodies (see page 33). These proliferations may lead to perforation of the bones, 

 even appearing beneath the scalp as small tumors which are reducible upon pressure ; or, in rare 

 instances, they may cause the bone to bulge, producing palpable, immovable, and hard bony 

 enlargements (in contradistinction to sebaceous cysts of the scalp which originate in the sebaceous 

 glands and are movable upon the underlying bone). (2) By the middle meningeal artery (see 

 page 29), which is deeply imbedded on the inner surface of the cranial wall, sometimes, how- 

 ever, reaching almost to the outer surface of the cranial vault or even perforating the external 

 table and producing marked external hemorrhage when injured. (3) Rarely by varicose dilata- 

 tions of the diploic veins (see below), which lead to atrophy of the bone, and also by the spheno- 

 parietal sinus (see page 32), which may be deeply imbedded in the cranial vault somewhat 

 posterior to the coronal suture. (4) By increasing age, which makes the skull considerably 



Internal plate 

 FIG. 2. A cross-section of the calvarium. 



thinner, lighter, and more fragile, so that places which were previously thin (e. g., the orbital 

 plate of the frontal, the lachrymal, the os planum of the ethmoid, the posterior cranial fossa, 

 the inner wall of the maxillary sinus) are converted into foramina by absorption. 



The sawed cross-section of the vertex reveals two compact layers of bone which enclose a 

 stratum of spongy tissue. The external plate averaging 1.5 mm. in thickness as well as 

 the internal plate about 0.5 mm. thick have numerous fine pores for the passage of the per- 

 iosteal vessels. There is no sharp line of division between either table and the enclosed diploe, 

 which is filled with red bone-marrow. Fractures of the inner table, such as the so-called stellate 

 fractures, or splinterings of the inner table without concomitant injury of the outer table are not 

 due to a greater brittleness of the inner table, but to its thinness and to its shorter radius of 

 curvature. 



The venous blood of the bone collects in the diploic veins which are situated in canals in the 

 diploe. The diploic veins are divided into the frontal, anterior temporal, posterior temporal, and 

 occipital. They are subject to considerable variation, and hemorrhage from them after injury 



