1338 CLINICAL AND TOPOGRAPHICAL ANATOMY 



B. The lower or vertical cells of the mastoid are developed later than is the 

 antrum, and vary much in their contents. The condition of the mastoid cells 

 varies ver}^ widely. They may be numerous (fig. 1088) or few. In the latter 

 case they are replaced by diploe, or by bone which is unusually dense, without 

 necessarily any pathological change. Hence mastoids have been classified as 

 pneumatic, diploetic, or sclerosed. 



As part of the surgical anatomy of this most important region, the different paths by which 

 infection of the tympanum and antrum may travel should be glanced at. The most important 

 are: — (1) Upward: either by advancing caries or by infection of veins going to the superior 

 petrosal sinus, or through the tegmina to the membranes; an abscess in the overlying temporal 

 lobe, usually the middle and back part. (2) Backward : the transverse (lateral) sinus and 

 cerebellum (abscess of the front and outer part of the lateral lobe) are reached in the same ways 

 as those given above, the mastoid vein being the one chiefly affected here. Macewen has shown 

 that the bony wall of the sinus, like those of the tegmina and the aqueduct of Fallopius, may be 

 naturally imperfect. (3) Downward : where the vertical cells are well developed (fig. 929) 

 mischief may reach the mastoid notch and cause deep-seated inflammation beneath the sterno- 

 mastoid. (v. Bezold's abscess.) (4) Lateralward: the explanation of this, in early life, has 

 been given above. (5) Medialward : the facial nerve, or by the fenestra ovahs; the labyrinth is 

 now in danger. When the internal ear and auditory nerve are affected, infection finds another 

 path to the cerebellar fossa. 



The sphenoidal sinuses are less important surgically, but these points should be remem- 

 bered: — (1) Fracture through them may lead to bleeding from the nose, which is thus brought 

 into communication with the middle fossa; (2) the communication of their mucous membrane 

 with that of the nose may explain the inveteracy of certain cases of polypi and ozajna; (3) here 

 and in the frontal sinuses very dense exostoses are sometimes formed. Before any operative 

 attack on these sinuses is undertaken, their most important relations should be remembered. 

 Thus above are the olfactory and optic nerves, the pituitary body, and front of the pons. 

 Externally lie the cavernous sinus and superior orbital (sphenoidal) fissure. Below is the roof 

 of the no.se. 



The ethmoidal and maxillary sinuses are considered later in connection with the Nose. 

 See also the sections on Osteology and Respiratory System. 



CRANIO-CEREBRAL TOPOGRAPHY 



To make as clear as possible the points of practical importance which have, of 

 late years, been put on a definite basis, and which the surgeon may have to recall 

 and act upon at very short notice, cranio-cerebral topography will be spoken of 

 under the following headings: A. Relation of the brain as a whole to the skull. 

 B. Relation of the chief sulci and gyri to the skull. C. Localisation of the 

 chief sulci and gyri. Before alluding to the above, it is necessary to say distinctly 

 that the following surface-markings and points of guidance are only approxi- 

 mately reliable, for the following reasons: (1) In two individuals of the same 

 age and sex the sulci and convolutions are never precisely alike. (2) The rela- 

 tions of the convolutions and sulci to the surface vary in different individuals. 

 (3) That as the surface area of the scalp and outer aspect of the skull are greater 

 than the surface area of the brain, and as the convexities do not tally, lines drawn 

 on the scalp or skull cannot always correspond precisely to cerebral convolutions 

 or sulci. It results from the above that when a definite area of the surface is said 

 to correspond accurately in any individual to a definite area of the brain surface, 

 this result has been correlated from many examinations; and that as surface- 

 markings, shape, and processes of skull and arrangement of surface are all liable 

 to variations in different individuals, the surgeon must allow for these variations 

 by removing more than that definite area of skull which is said to correspond 

 exactly to that part of the bruin which ho desires to expose. 



A. Relation of the brain as a whole to the skull (figs. 1089, 1091). — To trace 

 the lower level of each cerebral hemiaphere on the skull, the chalk would start from 

 the lower part of the glabella; thence the line representing the lower borders of the 

 frontal lobe pursu(!s a course, slightly curved upward, about 0.8 cm. (I in.) 

 above the supraorbital margin; next, cro.ssing the temporal crest about 1.2 cm. 

 {\ in) above the zygomatic (external angular) process, it passes not quite hori- 

 zontally but descending slightly to a point in the temporal fossa just below the 

 tip of the great wing of the splienoid (pterion), 2.5 cm. (1 in.) behind the zygo- 

 matic process. From this point the line of the level of the brain, now convex 

 forward and corresponding to the anterior extremity of the temporal lobe, 

 would dip down, still within the great wing of the sphenoid, to about the centre 

 of the zygoma. Thence the line of the hnver l)order of the temporal lobe would 



