250 Anatomy of Skeleton 



the basal ossifications. This equal extension is uncommon, however, for usually the further 

 extension is unequal : one sinus may, for example, involve the postsphenoid of its own side and then 

 extend to the postsphenoid of the other side, so that the other sinus remains small, confined to the 

 presphenoidal region, and the septum between them, median in front, swings to one side further 

 back where it corresponds with the condensation between the pre- and post-sphenoid. One or 

 both of the " presphenoidal " sinuses may extend laterally (instead of, or in addition to, the back- 

 ward growth) and so involve the neighbouring part of the great wing : in this case the backward 

 and the lateral extensions are separated by a partial septum corresponding with the condensation 

 of the lingula, and this septum, directed obliquely forwards and inwards, may be very prominent 

 if the region of the lingula has not been invaded, or more or less destroyed if such an invasion has 

 taken place. A septum of this sort lies under the carotid groove, as does also the outer part of a 

 transversely directed septum associated with the condensation between pre- and post-sphenoids, 

 and these might be called carotid buttresses within the cavity : they are usually better marked above 

 than below and are situated of course in the deeper parts of the cavities at their sides. Various 

 combinations of these extensions may be seen in different individuals, but in general, it may be 

 recognised that there is a septum, median in front but as a rule directed to one or other side behind, 

 with lateral and posterior loculi more or less separated by a carotid buttress : the deep and anterior 

 parts of the cavities may show a partial separation by an incomplete transverse septum marking 

 the plane of condensation between pre- and post-sphenoids, and this, like the deflected inter-sinus 

 septum, may run into a carotid buttress. Exceptionally the sinus may extend (a) into the great 

 wing, reaching as far, it may be, as the groove for the maxillary nerve, and even invading the base 

 of the outer pterygoid plate, passing over the Vidian nerve, (b) into the roots of the small wing, 

 the optic nerve and ophthalmic artery lying in a prominent bony canal projecting into the cavity, 

 or (c) into the basi-occiput, from which it is nearly always separated by the condensation between 

 this bone and the basisphenoid. 



The sinus begins its growth about the third or fourth month of mtra-uterine life, is at birth a 

 definite separate small cavity, in relation with the front aspect of the sphenoidal ossification, and 

 enclosed by its own turbinate, grows fairly rapidly in the next few years, so that a noticeable 

 cavity, invading the presphenoid, is present at five years of age, and takes on a more rapid exten- 

 sion at or before the age of ten, and again at puberty. 



The frontal sinus develops as an upgrowth from one of the groups of cells (anterior ethmoidal) 

 which form under cover of the overhanging anterior and upper end of the middle turbinal. The 

 enlarging cell extends slowly upwards, the extension beginning about the middle of intra-uterine 

 life. The growing cavity does not, as a rule, reach the frontal bone before birth, but invades that 

 bone within the first year, and grows steadily up to the ninth or tenth year : after this it seems to 

 take on a more rapid growth. The extensions are usually unequal, so that the septum between the 

 cavities is deviated in its upper part, though generally more or less median below. In metopic 

 skulls (p. 200) a sinus never transgresses the middle line. In other cases one sinus may completely 

 overshadow the other, so that it seems at first as if no septum were present. The opening of the 

 sinus depends on the site of its origin : if it is an enlarged member of the lateral group of anterior 

 cells, it opens into the top of the hiatus semilunaris, from which these cells take origin as out- 

 pouchings, but, if from more medially situated cells, as is perhaps more frequently the case, the 

 sinus does not open directly into the upper end of the hiatus. 



The adult sinus is extremely variable in form and extent. In a very general way it might be 

 described as roughly pyramidal, the apex being directed upwards, but it is often rounded or ovoid, 

 and may present a partial subdivision : this subdivision is in some cases really due to the simul- 

 taneous upgrowth of two cells which have partly coalesced. The front wall of the sinus is the 

 thickest wall, and contains diploic tissue, the posterior wall being made of compact bone. The 

 floor shows a general slope downwards and inwards towards the opening in its hinder and inner 

 part, but there is often a depressed fossa in front of this, and the floor is generally uneven. It lies 

 over the inner part of the orbital roof, not often going further than, or even so far as, the line of the 

 supra-orbital nerve, and covers the anterior ethmoidal cells internally. The amount of its back- 

 ward extension, greatest internally, is very variable, but it seldom reaches the depth of an inch : 

 an average measurement in this direction would be in the neighbourhood of f inch. 



The maxillary sinus (antrum of Highmore, maxillary anlrum) is a cavity situated in the 

 maxilla, of an irregular pyramidal shape, with its base inwards. Its front wall is fairly thick, and 

 is made by the facial surface of the maxilla, its floor is formed by the thick alveolar portion of the 

 bone, and its roof and inner back wall are thin as is its inner wall, which is made of the inner 

 lamina of the maxilla, overlaid by the lower end of the lachrymal, the uncinate process of the eth- 



