1364 SUEFACE AND SUKGICAL ANATOMY. 



between it and the posterior surface of the petrous portion of the temporal bone as far 

 as the acoustic nerve, which occupies the angle between the cerebellum and the pons. 



Meningeal Arteries. When the calvaria is removed the meningeal arteries 

 are found to adhere firmly to the dura. Of these vessels the middle meningeal 

 artery is the only one of surgical importance. It is frequently lacerated in 

 fractures of the skull ; the blood is generally extra vasated between the dura and 

 the bone, and the bleeding point lies beneath the clot. After entering the cranial 

 cavity through the foramen spinosum, the main trunk, which is usually about 

 1 \ in. in length, runs laterally and slightly forwards to bifurcate into anterior and 

 posterior divisions at a point a finger's-breadth above the middle of the zygomatic 

 arch, viz., at or close behind the point N, Fig. 1069. When the main -trunk is 

 short the bifurcation takes place opposite the middle of the zygomatic arch. 



The anterior and larger division passes upwards, with a slight forward con- 

 vexity, a little behind the spheno-squamosal suture and across the pterion to 

 the sphenoid angle of the parietal bone. From that point the vessel is continued 

 upwards and slightly backwards, behind the coronal suture ; it gives off branches 

 which ascend over the motor area. The. position and general direction of the 

 anterior branch may be said to correspond to the line MN ; it follows, therefore, 

 that the artery will be encountered in trephining over the lower and anterior part 

 of the Kolandic area, especially over the motor centres for the tongue and face. 



The posterior division passes almost horizontally backwards, towards the 

 mastoid angle of the parietal bone. 



To expose the trunk of the vessel and its bifurcation, the trephine is applied 

 immediately above the middle of the zygomatic arch. To expose the anterior 

 division the pin of the trephine may be applied at the point A, which strikes the 

 artery as it crosses the pterion and grooves the sphenoidal angle of the parietal 

 bone. The inferior segment of the disc of bone removed is much thicker than the 

 superior, as it involves the prominent ridge which passes from the tip of the great 

 wing of the sphenoid on to the sphenoidal angle of the parietal bone. At the 

 sphenoidal angle of the parietal bone, the artery frequently runs in a canal for a 

 distance of half an inch. It follows, therefore, that a considerable thickness of 

 bone has to be sawn through at the inferior segment of the circle before the disc 

 can be removed, and during the removal bleeding may occur from the artery as 

 it lies in the canal. 



Vogt localises the anterior division at a point a thumb's-breadth behind the 

 tubercle on the posterior border of the zygomatic bone and two fingers'-breadth above 

 the zygoma. Kronlein trephines at a point 1J in. behind the zygomatic process of 

 the frontal, on a line drawn from the supra-orbital margin backwards parallel to 

 Keid's base-line. If the centre of the trephine be placed at the mid-point of the 

 lower third of the line MA. the anterior division will be reached above the canal and 

 the ridge at the sphenoidal angle of the parietal ; if the bleeding-point is lower 

 down, the trephine opening may be enlarged downwards along the line AN. 



The course of the posterior division may be indicated upon the surface by draw- 

 ing a line backwards from the point N parallel to PK, that is to say, a finger's- 

 breadth above the zygoma and the supra-mastoid crest. 



When the frontal branch of the anterior division is injured, the clot is in the fronto-temporal 

 region, and involves more especially the motor area for the face, and, on the left side, Broca's con- 

 volution ; when the anterior division is wounded, the clot, which is larger, involves the parieto- 

 temporal region, and the motor symptoms are due to pressure upon the centres for the arm and 

 face ; in injuries to the posterior division the clot overlies the parieto-occipital region, and the 

 localising symptoms are sensory (Kronlein). In more extensive meningeal haemorrhage the clot 

 may cover the" greater part of the hemisphere. 



The superior sagittal sinus, which enlarges as it extends backwards, occupies the 

 median plane of the vertex from the glabella to the internal occipital protuberance, 

 where it opens into the confluens sinuum, and becomes continuous usually with the 

 right transverse sinus. Opening into the sinus, especially in the posterior part of 

 the parietal region, are the para-sinoidal sinuses, into which arachnoideal granulations 

 project. In opening the skull over the posterior part of the vertex, the edge of the 

 trephine should be kept at least three-quarters of an inch from the median plane. 



