chap, i.i THE SCALP. i \ 



be replaced in the opening, and will serve to make 

 good the gap left by the operation. 



In trephining the skull generally, the comparative 

 thickness of the cranial wall in various parts should 

 be borne in mind (page 28), and the large arteries of 

 the scalp should be avoided if possible. In order to 

 accommodate the instrument to the varying thickness 

 of the skull, the pin of the trephine is not allowed to 

 protrude more than -^ of an inch. The trephine 

 should not be applied over the frontal sinuses, which 

 are often of large size in the aged, and should, when 

 possible, keep clear of the sutures, owing to the 

 frequent exit of emissary veins at or about suture 

 lines. The bone, moreover, at certain of these lines 

 is of unequal thickness. Between the bones forming 

 the sutures passes the sutural membrane. This 

 structure blends with the dura mater, and laceration 

 of it may conduce to meningitis. Especially must 

 the superior longitudinal sinus be avoided, which 

 runs backwards in the middle line, and the lateral 

 sinus, whose course is represented by a line drawn 

 horizontally from the occipital protuberance to a point 

 about one inch behind the external meatus of the ear, 

 where it turns downwards to groove the mastoid process. 



The zygoma may be broken by direct or indirect 

 violence. In the latter case the violence is such as 

 tends to thrust the upper jaw or malar bone back- 

 wards. When due to direct violence, a fragment may 

 be driven into the temporal muscle, and much pain 

 caused in moving the jaw. In ordinary cases there is 

 little or no displacement, since to both fragments the 

 temporal fascia is attached above and the masseter below. 



The vessels and nerves of the scalp. The 

 supraorbital artery and nerve pass vertically upwards 

 from the supraorbital notch, which is situate at the 

 junction of the middle with the inner third of the 

 upper orbital margin. Nearer the middle line the 



