94 APPLIED ANATOMY. 



brain or dura directly over the middle-ear cavity, then one trephines above this line 

 or suprameatal crest, the lower edge of the trephine opening' being .5 cm. above it. 

 This will lead to the middle fossa of the skull, occupied by the temporosphenoidal 

 lobe. The sharp upper and posterior edge of the petrous portion of the temporal 

 bone gives attachment to the tentorium and separates the middle cerebral fossa in 

 front from the posterior fossa, containing the cerebellum, behind. The point at which 

 this ridge and tentorium reach the side of the skull is indicated by the point of cross- 

 ing of a line drawn up from the tip of the mastoid process, midway between its anterior 

 and posterior borders, and the line of the posterior root of the zygoma. The course 

 of the lateral sinus is indicated by a curved line from above and to the right (about 

 . 5 to i cm. ) of the external occipital protuberance to the upper posterior portion of 

 the mastoid process and thence to its tip. The anterior edge of the lateral sinus 

 reaches as far forward as a line drawn from the tip of the mastoid upward, midway 

 between its anterior and posterior borders. The point at which it turns is where this 

 mastoid line intersects the line of the zygoma. Its upper edge rises above this line 

 approximately i cm. The sinus is I cm. in width. The distance of the sinus from 

 the surface varies from .5 cm., or even less, at the top of the mastoid process to 1.5 

 cm. at its tip. So uncertain is this that the only safe way to expose the sinus is to 

 cut the bone off with a mallet and gouge in thin chips parallel to the surface. The 

 use of a trephine or other boring instrument is not to be advised. If the infection 

 of the lateral sinus has extended to the jugular vein this latter must be reached by 

 means of a separate incision in the neck. 



THE NOSE. 



Externally the nose forms a prominent projection on the face, hence it is fre- 

 quently injured and its construction should be studied in relation to those injuries. 

 It forms a conspicuous portion of the features, hence-deformities or disfigurements of 

 it are very distressing, so that plastic operations are done for their relief. Internally, 

 the nasal cavities are concerned in the sense of smell and form the passage-way to 

 and from the lungs and the various accessory cavities for the air in respiration. It 

 likewise serves as a receptacle for the tears as they come down the lachrymonasal 

 duct. Interference with the flow of air by obstruction of the nasal chambers may 

 cause affections of the pharynx, larynx, lungs, ears, or accessory sinuses ethmoid, 

 sphenoid, maxillary, and frontal. Catarrhal troubles may start in the nose and invade 

 any of these parts. They may even extend up the Eustachian tube and cause deaf- 

 ness; or up the lachrymonasal duct and cause trouble with the lachrymal canal or 

 conjunctiva. A knowledge of the nose is essential to all those who wish to devote 

 themselves especially to affections of the eye, ear, and throat, because the origin of 

 the affections of these organs may be in the nasal chambers instead of the organ in 

 which they are most manifest. 



The skin over the root of the nose is thin and lax. It is well supplied with 

 blood by the frontal and nasal branches of the ophthalmic, and the angular branch 

 of the facial arteries. In reconstructing a nose by means of a flap taken from the 

 forehead, it is these branches that nourish it. The laxity of the skin allows the 

 pedicle to be twisted around without interfering with the circulation. 



The skin over the tip and alae is thick and adherent to the cartilages. It pos- 

 sesses a comparatively scanty blood supply, hence its liability to suffer from cold, and 

 is a favorite site for ulcerations, as lupus, superficial epithelioma (rodent ulcer), etc. 

 Sebaceous and sweat glands are abundant, and stiff hairs guard the inside of the nos- 

 trils. These latter are not seldom the seat of small furuncles or boils, which are 

 extremely painful. This is due to the tension caused by .the congestion and swelling, 

 which is restricted by the tissues being so firmly bound to the cartilages beneath. 



Nerves. In addition to the olfactory nerve, the nose is supplied by the nasal, 

 infratrochlear, and infra-orbital branches of the fifth nerve, hence the eyes water 

 when the nose is injured. In certain cases of neuralgia affecting the ophthalmic 

 division of the fifth nerve, pain is felt along the side of the nose. As the nasal 

 nerve enters the skull from the orbit through the anterior ethmoidal foramen, it may 

 be involved in disease of the ethmoidal sinuses. 



