68 THE SKULL AS A WHOLE. 



between the petrous and squamous portions is the back part of the great wing of the 

 sphenoid bone. Between this division of the base of the skull and the palate are 

 the posterior nares, separated by the vomer, and bounded above by the body of the 

 sphenoid bone, below by the horizontal plates of the palate bones, and laterally by 

 the internal pterygoid plates. On each side of the posterior nares is the pterygoid 

 fossa, completed below by the tuberosity of the palate bone ; and a line from the 

 external pterygoid plate to the spine of the sphenoid forms the division between this 

 region and the zygomatic fossa. Immediately behind or internal to this line is the 

 groove for the cartilaginous part of the Eustachian tube, formed by the margins of 

 the great wing of the sphenoid and the petrous, and leading to the osseous part of the 

 tube in the temporal bone. Between the apex of the petrous, the basilar process, 

 and the sphenoid is the foramen lacerum ; in a line proceeding backwards and out- 

 wards from this are the free surface of the petrous, the lower orifice of the carotid 

 canal, the vaginal and styloid processes, and the stylo-mastoid foramen ; while 

 internal to these are the jugular and anterior condylar foramina. 



Between the basilar process of the occipital bone and the petrous portion of the temporal 

 is an irregular cleft, extending from the foramen lacerum backwards and outwards to the 

 jugular foramen, and called the petro-basilar fissure. This interval, together with the lower 

 part of the foramen lacerum, is filled in the recent state by fibrous tissue which often 

 contains one or two small Wormian ossicles. In front of the petrous portion, at the bottom 

 of the groove of the Eustachian tube, is the petro-splienoidal fissure, also continued outwards 

 from the foramen lacerum. 



The posterior division presents on each side of the foramen magnum, from 

 within outwards, the occipital condyle, the under surface of the jugular process, the 

 occipital groove of the temporal bone, the digastric fossa, and the mastoid process. 

 Behind the foramen magnum is the tabular part of the occipital bone, with its ridges 

 and muscular impressions. 



THE INTERIOR OF THE CRANIUM. 



The wall of the cranium consists of two layers of compact bony substance, the 

 outer and inner tables, and an intervening cancellated substance, called diploe. The 

 inner or vitreous table has a smooth, close-grained, shining appearance, is hard and 

 brittle, and presents irregular digitate impressions corresponding to the convolutions 

 of the cerebrum. The thickness of the skull-cap or calvaria is fairly uniform, and 

 generally ranges from one-sixth to one-fourth of an inch : it is somewhat increased 

 along the middle line, especially in front and behind, and diminishes below on each 

 side, in the temporal fossa. The base of the skull varies greatly in this respect: 

 the thickest parts are the basilar process, the petrous and mastoid portions of the 

 temporal bones, and the occipital bone at the protuberances and ridges. The 

 thinnest portions of the cranial wall are the cribriform plate of the ethmoid and the 

 orbital plates of the frontal bone, in both of which the diploe is absent ; the bone is 

 also thin and compact in the middle part of the inferior occipital fossse, and in the 

 lower part of the squama and the glenoid fossa of the temporal. 



The upper part of the cranial cavity is enclosed by a single vaulted dome, formed 

 by the frontal, parietal, and occipital bones. This is marked on its internal surface 

 by the groove for the superior longitudinal sinus, by shallow cerebral impressions, by 

 small ramified meningeal grooves, and by Pacchionian fossae of varying depth. The 

 only apertures in the roof of the skull are the inconstant parietal foramina, which 

 open by the side of the longitudinal sulcus posteriorly ; they give passage to emissary 

 veins, and occasionally a branch of the occipital artery. 



The internal base of the skull is divided into three fossae, named anterior, 

 middle, and posterior. 



