MEDIAN SAGITTAL SECTION OF THE SKULL. 183 



temporal, the inferior petrosal lying along the line of suture between the petrous 

 part of the temporal and the basilar part of the occipital bone; the occipital 

 sinus grooving the internal occipital crest ; and the transverse sinus curving for- 

 wards and laterally from the internal occipital protuberance, across the cerebral 

 surface of the squamous part of the occipital, to reach the mastoid angle of the 

 parietal bone, in front of which it turns downwards and medially to reach the 

 jugular foramen, describing a sigmoid curve, and grooving deeply the inner surface 

 of the mastoid and posterior aspect of the petrous portions of the temporal bone. 

 Before it terminates at the jugular foramen it again reaches the occipital bone and 

 channels the upper surface of the jugular process of that bone. Slight grooves for 

 meningeal arteries are also seen some pass upwards, whilst others turn downwards 

 and are occupied by branches from the posterior offsets of the middle meningeal 

 arteries. 



Median Sagittal Section of the Skull. 



Such a section should be made a little to one or other side of the median plane, so as to pass 

 through the nasal cavity lateral to the septum ; one-half will then display the nasal septum in 

 position, whilst in the other the lateral wall of the nasal cavity of that side will be exposed. 



The form of the cranial cavity is, of course, subject to many variations dependent 

 on individual and racial peculiarities. The following details are, however, worthy 

 of note. The posterior border of the foramen magnum (opisthion), and consequently 

 the floor of the posterior cranial fossa, occupies the same horizontal plane as the 

 hard palate. The anterior border of the foramen magnum (basion) lies a little 

 higher, so that the plane of the foramen is, in the higher races at least, oblique, and 

 is directed downwards and slightly forwards. From the basion a line passing 

 upwards and forwards to reach the suture between the sphenoid and ethmoid 

 passes through the basi-cranial axis formed by the basi-occipital, the basi-sphenoid, 

 and the presphenoid. The basi-cranial axis is wedge-shaped on section posteriorly, 

 whilst anteriorly it is of considerable width, and has within it the large sphenoidal 

 air sinus. Its upper surface leads upwards and forwards with a varying degree of 

 obliquity from the basion to the overhanging edge of the dorsum sellse, in front of 

 which the sella turcica, the floor of which is quite thin, is well seen in the section. 



From the tuberculum sellse the floor of the anterior fossa follows a more or less 

 horizontal direction, corresponding pretty closely to the level of the axis of the orbital 

 cavity. The roof of the orbit is seen to bulge upwards to a considerable extent into the 

 floor of the anterior fossa ; whilst the floor of the middle fossa sinks to a level corresponding 

 to that of the under surface of the basi-cranial axis, where it forms the roof of the choanae. 

 The maximum length of the skull is measured from the glabella (a point between 

 the superciliary arches) to the occipital point posteriorly. It is noteworthy that the 

 maximum occipital point does not necessarily correspond to the external occipital pro- 

 tuberance (inion). The greatest vertical height usually corresponds to the distance from 

 the basion to the bregma (point of union of the sagittal with the coronal suture), though 

 to this rule there are many exceptions. On looking into the posterior fossa the hypo- 

 glossal canals and jugular foramina and the internal acoustic meatus are seen in line, 

 sloping from below upwards. The internal acoustic meatus lies in a vertical plane, 

 passing through the basion. The grooves for the middle meningeal artery and its 

 branches are very obvious. The anterior groove curves forwards and laterally, and 

 reaching the cerebral surface of the pterion, passes towards the vertex at a variable 

 distance behind and more or less parallel to the coronal suture. From this grooves pass 

 forwards across the suture to reach the frontal bone. Another groove curves upwards 

 and backwards a little below the line of the parieto-squamosal suture. From this an 

 upwardly directed branch radiates on the cerebral surface of the parietal bone, in the 

 region of the parietal tuberosity, whilst a lower branch passes backwards some little 

 distance above the lambdoid suture, and gives offsets which curve downwards and 

 medially over the cerebral surface of the squama occipitalis of the occipital bone. 



Cavum Nasi. In the section through the nasal cavity the structures which form 

 its lateral wall can now be studied. These are the nasal bone ; the frontal process 

 of the maxilla ; the lacrimal bone ; the labyrinth of the ethmoid, comprising the 

 superior and middle conchal bones ; the perpendicular part of the palate bone ; 

 the inferior conchal bone ; and the medial surface of the medial pterygoid lamina. 



