1164 SURFACE AND SURGICAL ANATOMY. 
Keen drains the ventricles through an opening 14+ in. behind the external 
auditory meatus and the same distance above Reid’s base line (a lne drawn 
backwards from the lower margin of the orbit through the centre of the external 
auditory meatus), the instrument being passed into the brain towards the summit 
of the opposite auricle. If the ventricle be not distended it will be reached at a 
depth of two inches from the surface. 
To open the subarachnoid space, the pin of a small trephine is placed over the 
mid-point of the line EA; the subarachnoid is incised as it crosses the stem of the 
fissure of Sylvius from the frontal lobe to the anterior extremity of the temporal 
lobe. Care must be taken to keep in front of the middle meningeal artery. 
The cisterna magna, situated between the back part of the under surface of the 
cerebellum and the medulla oblongata, may be reached by turning down a flap 
of soft parts, and removing a circle of bone a little above the foramen magnum, 
and immediately to one side of the middle hne so as to avoid the occipital sinus. 
The fourth ventricle may be opened up by making a somewhat larger trephine 
opening in the mesial plane and separating the posterior extremities of the 
tonsillar lobes of the cerebellum. 
To expose the lateral hemisphere of the cerebellum, trephine over the centre of 
a line drawn from the tip of the mastoid process to the external occipital pro- 
tuberance. The occipital artery and the mastoid emissary vein will be divided 
in turning down the flap. 
Meningeal Arteries.—When the calvarium 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 extravasated 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 
14 in. in length, runs outward 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. When the main trunk is short the 
bifureation is situated lower down and farther back. 
The anterior and larger division passes upwards, with a shght forward con- 
vexity, a little behind the spheno-squamous suture and across the pterion to 
the anterior inferior angle of the parietal bone. From this 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 readily mapped out on the surface, as 
it corresponds to the lower two-thirds of the precentral lme MN; it follows, 
therefore, that the artery will be encountered in trephining over the lower and 
anterior part of the Rolandic area, especially over the motor centres for the tongue 
and face. 
The posterior division passes almost horizontally backwards towards the 
posterior inferior 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 anterior inferior angle of the parietal 
bone. The lower segment of the disc of bone removed is much thicker than the upper, 
as it involves the prominent ridge which passes from the tip of the great wing of the 
sphenoid on to the anterior inferior angle of the parietal bone. At the anterior 
inferior 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 lower segment of the circle before the dise can 
be removed, and during the removal bleeding may occur from the artery as it les 
in the canal. 
Vogt localises the anterior division at a point a thumbh’s-breadth behind the 
tubercle on the posterior border of the malar bone and two fingers’-breadth above 
the zygoma. Kronlein trephines at a point 14 in. behind the external angular 
process, on a line drawn from the supra-orbital margin backwards parallel to 
