THE BONES OF THE HEAD 149 



from this foramen are the occipital condyle, jugular process, 

 occipital groove for the occipital artery, digastric groove for the 

 posterior belly of the digastric, and the mastoid process. Behind 

 the occipital condyle is the posterior condylar fossa, in which 

 there may be a posterior condylar foramen for the passage of 

 an emissary vein from the lateral sinus. Behind the foramen 

 magnum is the supra-occipital portion of the occipital, which presents 

 the external occipital crest in the median line, and the inferior curved 

 line extending outwards on either side from its centre. 



The Interior of the Cranium. 



A sagittal or antero-posterior section of the skull a little to one 

 side of the median plane shows the septum nasi alreadj'^ described. 

 Along, and at either side of, the vault of the cranium is the groove 

 for the superior longitudinal venous sinus, which extends from 

 before backwards, and on either side of its parietal portion are the 

 Pacchionian depressions. The internal openings of the parietal 

 foramina may be seen, as well as the branching system of menin- 

 geal grooves, and digitate impressions. The basi-cranial, basi- 

 facial, and basi-bregmatic axes are to be studied from this section. 

 The basi-cranial axis represents a line drawn upwards and forwards 

 from the basion to the spheno-ethmoidal suture. The basi-facial 

 axis corresponds ^vith a line dra\vn from the spheno-ethmoidal 

 suture to the subnasal point. The angle formed by these tvvo 

 axes is known as the cranio-facial angle. The basi-bregmatic 

 axis represents a line drawn vertically from the basion to the 

 bregma. 



The most instructive coronal or transverse section is one made 

 in the plane of the basi-bregmatic axis. Such a section gives 

 important views of the parts within the petrous portion of the 

 temporal, such as the external auditory meatus, tympanum, and 

 vestibule. 



When a horizontal section has been made on a level with the occi- 

 pital point and the most prominent part of the glabella, the vaulted 

 roof of the cranium is removed. This is called the calvaria, 

 or skull-cap, and it is formed by portions of the frontal, parietals, 

 squamous portions of the temporals, and occipital. The outer 

 plate is strong, except over the temporal region, but the inner is 

 brittle and readily cracked, from which circumstance it is known 

 as the vitreous (glassy) plate. Between the two plates there is 

 cancellated tissue, here called diploe. The interior of the calvaria 

 presents branching meningeal grooves, digitate impressions, and 

 along the middle line the groove for the superior longitudinal venous 

 ~ nus, with depressions at either side for the Pacchionian bodies. 

 The openings of the parietal foramina may be noted. 



