THE MORPHOLOGY OF THE SKULL 117 



The posterior fossa is marked by several sutures, viz., petro-occipital, occipito-mastoid, 

 parieto-mastoid, and in young skulls the basilar (occipito-sphenoidal). In addition, the follow- 

 ing points may be noted: — 



The clivus, extending from the dorsum sellae to the anterior margin of the foramen magnum, 

 and in relation with the basilar artery, the pons, the medulla, the sixth nerves, and the basilar 

 sinUs. 



The foramen magnum, occupied in the recent state by the lower end of the medujla oblon- 

 gata and its membranes, the vertebral, anterior spinal and posterior spinal arteries, the accessory 

 (eleventh) cranial nerves, and the tectorial membrane. 



The hypoglossal canal (foramen) , sometimes divided by a spicule of bone into two divisions, 

 for the two parts of the hypoglossal nerve and a meningeal branch of the ascending pharyngeal 

 artery. 



The internal occipital crest, behind the foramen magnum, for the attachment of the falx 

 cerebelli. It sometimes presents a depression known as the vermiform fossa. 



The internal auditory meatus, for the seventh and eighth cranial nerves, the pars inter- 

 media, and the internal auditory vessels. 



The jugular foramen (foramen lacerum posterius), somewhat pyriform in shape, and divis- 

 ible into three compartments. The anterior division, placed somewhat medially, transmits 

 the inferior petrosal sinus and is sometimes completely separated by an intra-j ugular process of 

 bone; the middle division transmits three cranial nerves, the ninth, tenth, and eleventh; and, in 

 the posterior division, placed somewhat laterally, the transverse sinus becomes continuous with the 

 internal jugular vein. A meningeal branch of the ascending pharyngeal or occipital artery 

 enters the cranium through this division of the foramen. 



The termination of the groove for the transverse sinus with the internal orifice of the mas- 

 toid foramen. 



The aquaeductus vestibuli and the fossa subarcuata, on the posterior surface of the petrous 

 portion of the temporal. 



The cranium of an average European has a capacity of 1450 c.c. The circumference, taken 

 in a plane passing through the ophryon in front, the occipital point behind, and the pterion at 

 the side, is 52 cm. The length from the ophryon to the occipital point is 17 cm., and the width 

 between the parietals at the level of the zygomata is 12.5 cm. The proportion of the greatest 

 width to the length is known as the cephalic index, i. e., index of breadth. A skull with an aver- 

 age cephalic index is mesaticephalic. When the index is above the average, it is brachycephalic 

 (short and broad), and when below the average, dolichocephalic (long and narrow). The height 

 from the basion to the bregma is nearly the same as the width at the level of the zygomata. 

 The cranio-facial angle is about 96°. 



THE MORPHOLOGY OF THE SKULL 



In man the skull during development passes through three stages. At first the brain vesi- 

 cles are enclosed in a sac of indifferent tissue which ultimately becomes tough and fibrous to form 

 the membranous cranium. This, in turn, is partly converted into the membrane or roof bones 

 of the cranium, whilst the remainder is represented in the adult by the dura mater. At the 

 sides and base of the membranous cranium, however, cartilage is deposited, chondro -cranium, 

 in which, as well as in the membranous tracts, osseous tissue appears in due course. Eventually, 

 as osseous box is formed, consisting of membrane bones and cartilage bones intricately 

 interwoven. 



A study of the skull in the chondral stage is very instructive. It consists of two parts: 

 (1) The skull proper and (2) the appendicular elements. 



(1) The skull proper consists of three regions: — 



(a) The notochordal region, which ultimately gives rise to the chief parts of the occipital 

 bone and a part of the sphenoid. It is named notochordal because the notochord runs 

 in it as far as the anterior extremitj', i. e., the level of the fossa hypophyseos (sella turcica.) 



(b) Anterior to the notochordal is the trabecular region, from which the remainder of the 

 sphenoid is developed. 



(c) The most anterior part of the prechordal portion of the base is the ethmo -vomerine 

 region, from which the nasal septum and its cartilages arise. These three parts continue 

 forward the line of the vertebral axis, and constitute a cranio-facial axis terminating, in 

 front, in the premaxillae. Finally, wedged in on each side, between the notochordal and 

 trabecular regions, is the complicated periotic capsule. 



The chondro -cranium at the thii-d month presents the following parts. Seen from above, 

 the cartilage extends from the cranial base to a spot midway between the base and the vertex, 

 shading off indefinitely on the membranous :vall. The oval masses on each side are the periotic 

 cartilages, in which the fossae subarcuatse are conspicuous objects. Each periotic cartilage is 

 joined to the sphenoid by a strip, termed the sphenotic cartilage, which usually persists in the 

 adult skull. The cartilage for the orbito-sphenoid (the small wing) is co-extensive with the 

 ali-sphenoid, and forms part of the lateral wall of the skull. The snout-like appearance of the 

 anterior part of the skull is caused by the fronto-nasal plate. On each side of the ethino- vomerine 

 plate, near its anterior termination, are two small concave pieces of cartilage for Jacobson's 

 organs. They are sometimes referred to as the ploughshare cartilages, owing to their shape. 

 Further details are given in fig. 141. 



(2) The appendicular elements of the skull are a number of cartilaginous rods surrounding 

 the visceral cavity — i. e., nose, mouth, and pharynx — which undergo a remarkable metamor- 

 phosis, and are represented in the adult by the ear bones, the styloid process, and the hyoid bone. 



