THE CRANIUM. 1165 
Reid’s base line. If the centre of the trephine be placed at the mid-point of the 
line LA the anterior division will be reached above the canal and the ridge at the 
anterior inferior angle of the parietal; should the bleeding-point be lower down 
the trephine opening may be enlarged downwards along the line MA. 
The course of the posterior division may be indicated upon the surface by draw- 
ing a line backwards from the point M parallel to PR, 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 
race 5 1 injuries to the posterior division the clot overlies the parieto-occipital region, and the 
localismg symptoms are sensory (Kronlein). In more extensive meningeal hemorrhage the clot 
may cover the greater part of the hemisphere. 
The superior longitudinal sinus, which enlarges as it extends backwards, occupies 
the mesial plane of the vertex from the glabella to the internal occipital pro- 
tuberance, where it opens into the torcular Herophili, and becomes continuous 
usually with the right lateral sinus. Opening into the sinus, especially in the 
posterior part of the parietal region, are the para-sinoidal sinuses, into which 
Pacchionian glands 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 mesial plane. 
The lateral sinus may be mapped out on the surface by drawing a line, shghtly 
convex upwards, from a point a little above the level of the external occipital pro- 
tuberance to the posterior inferior angle of the parietal bone, at, or a little in front 
of, the point R, which forms the highest part of the arch of the sinus; from this 
point the upper border of the sinus follows the line PR for a distance of one inch, 
and then curves downwards and forwards to a point # in. below and behind the 
centre of the external auditory meatus, where it finally curves inwards and 
forwards to open into the jugular bulb, which occupies the jugular foramen. The 
anterior border of the descending or mastoid portion of the simus may be mapped 
out by drawing a line VW from a point a finger’s-breadth behind the post- 
auricular point of the temporal crest to the anterior border of the tip of the 
mastoid process. In wounds of the sinus the hemorrhage is very free, owing to 
the inability of its walls to collapse, but the bleeding is easily controlled by 
plugging. 
Of the cerebral arteries, the middle supplies almost the whole of the motor area, 
and one of its lenticulo-striate branches, which enters the brain at the anterior 
perforated space, is called “the artery of cerebral hemorrhage” trom the frequency 
of its rupture in apoplexy. The extravasated blood involves the motor part of the 
internal capsule. The postero-mesial central branches of the posterior cerebral 
artery, which enter the brain at the posterior perforated spot, supply the optic 
thalamus and walls of the third ventricle; hemorrhage from one of these 
branches is apt to rupture into the ventricle. The postero-lateral central branches 
of the posterior cerebral artery supply the optic thalamus, and when one of these 
vessels ruptures the hemorrhage is apt to invade the posterior or sensory part of 
the interna] capsule. 
Kar.—The skin covering the outer surface of the auricle is tightly bound 
down to the perichondrium, hence inflammations of it are attended with little 
swelling but much pain. The posterior auricular artery, which ascends along the 
groove ‘at the posterior attachment of the auricle, is immediately anterior to the 
incision for opening the mastoid antrum. 
The external auditory canal, the general direction of which is downwards, for- 
wards, and inwards, possesses various curves of practical importance. The highest 
part of the upward convexity, which is also the narrowest part of the canal, is 
situated at the centre of its osseous portion ; beyond this the floor sinks to form a 
recess In which foreign bodies are liable to be imprisoned. Of the two horizontal 
curves the outer is convex forwards, the inner concave forwards. The skin of the 
osseous portion of the canal is thin and fused with the periosteum, hence when 
