24 SURGICAL APPLIED ANATOMY. [Chap. n. 



through a rupture in the tympanic membrane, and is 

 derived from the vessels of the tympanum and its 

 membrane, or from an intracranial extravasation, and 

 in some cases from a rupture of one of the sinuses 

 about the petrous bone. The blood may follow the 

 Eustachian tube, and may escape from the nose or 

 mouth, or be swallowed and subsequently vomited. 

 To allow of the escape of cerebro-spinal fluid by the 

 ear (" the serous discharge "), (A) the fracture must 

 have passed across the internal auditory meatus ; (B) 

 the tubular prolongation of the membranes in that 

 meatus must have been torn ; (c) there must be a com- 

 munication between the internal ear and the tympa- 

 num ; and (D) the membraiia tympana must have been 

 lacerated. (3) In fractures of the posterior fossa an 

 extravasation of blood may appear about the mastokl 

 process or at the nape of the neck, or may even 

 extend into the cervical region. 



It may be added that in compound fractures of the 

 vault associated with tearing of the dura mater and 

 arachnoid, an escape of cerebro-spinal fluid has in a 

 few rare instances been noted, and that in cases of 

 free serous discharge from the ear after injury to the 

 head, but without fracture, the fluid is derived from 

 the mastoid cells, and escapes through a rupture in 

 the tympanic membrane. 



The thickness of the skull cap varies greatly, 

 not only in different parts of the same skull, but also 

 in corresponding parts in different individuals. The 

 average thickness is one-fifth of an inch. The thickest 

 parts are at the occipital protuberance (where the 

 section may measure half an inch), the mastoid pro- 

 cess, and the lower part of the frontal bone. The 

 bone over the inferior occipital fossae is very thin, 

 while it is thinnest over the squamous bone. Here 

 the bone may be no thicker in parts than a visiting 

 card. The skull is also thinned over the sinuses and 



