2 8 TOPOGRAPHIC AND APPLIED ANATOMY. 



FIG. 7. The layers of the scalp, the cranium, and the membranes of the brain. 



FIG. 8. The sinuses of the dura mater. The calvarium has been removed with the exception of a central 

 arch of bone, so that the processes of the dura mater have been left intact. 



4. The occipitalis minor nerve from the cervical plexus, and 



5. The occipitalis major, the nerve of cervical neuralgia, not a branch of the cervical plexus 

 but a dorsal branch of the second cervical nerve. It usually perforates the fibrous origin of the 

 trapezius in company with the occipital artery and immediately divides into several branches 

 which ascend to the vertex. 



The exposure of the nerve is difficult, as it is imbedded in dense connective tissue and the 

 pulsations of the occipital artery, the most frequently recommended guide, are often not dis- 

 tinctly palpable. 



The external periosteum of the skull, the pericranium, is relatively thin in comparison with 

 the internal periosteum (dura mater). It is very vascular and is not closely attached to the bones 

 except at the sutures, where it becomes continuous with the outer layer of the dura mater constitut- 

 ing the inter-sutural membrane. During childhood the pericranium lies loosely upon the skull- 

 cap, and in cephalhaematoma neonatorum it is frequently stripped up from the bones (particu- 

 larly from the parietal) as far as the surrounding sutures. 



In flap-like wounds of the scalp the pericranium is easily stripped up from the bone [except 

 at the sutures, where it is firmly attached. ED.] in association with the overlying tissues. It 

 receives its blood-supply chiefly from the vessels of the scalp, but a portion of it is also furnished 

 by the perforating branches of the lateral or endosteal layer of the dura mater (see page 29). [The 

 pericranium differs from the periosteum of other bones in that it may be removed for a consider- 

 able area without producing necrosis, due to the fact that the cranial bones are in large part 

 nourished by vessels from the outer layer of the dura. ED.] 



THE MEMBRANES OF THE BRAIN. 



The dura mater is of twofold importance: (i) For the skull; (2) for the brain. 



i. The dura mater forms the internal periosteum of the cranium. Since it is responsible for 

 the growth in thickness of the bone, we find it intimately adherent to the cranial vault during 

 childhood, while in the adult after this growth is completed, the cranial cavity may be opened and 

 the calvarium removed, leaving the dura mater and the underlying brain intact. Even then, 

 however, the numerous vessels passing from the dura into the bones are torn across. In general 

 the dura is more intimately attached to the base than to the cranial vault, the adhesion being most 

 firm at the cribriform plate of the ethmoid, the sella turcica, the clivus, and the petrous portion of 

 the temporal bone [which in part explains the infrequency of extra-dural collections of blood 

 and pus at these situations. ED.]. If the excised dura is placed in water, numerous vessels are 

 seen upon its outer surface, while the inner surface seems smooth and contains only capillary 

 ramifications. It is a fact that no vessels pass from the dura to the brain. It is completely 

 separated from the underlying pia mater by the subdural space, which is filled with lymph. The 

 importance of the dura as a periosteal layer is also distinctly demonstrated by the occasional 

 ossification of this structure. [By most authorities the dura mater is said to consist of two layers, 



