THE SKELETON 



17 



thin, dense plates, separated by a considerable 



thickness of bone of a spongy texture. The 



spaces of the spongy bone contain air derived 



from the auditive tubes 



of Eustachius. A very 



noticeable feature of the 



skull is the presence of y.- 



a large, hemispherical ^•*"' 



tympanic cavity on each 



side of the hinder part 



of the cranium. The 



cavity forms the drum 



of the ear when the soft 



structures of the head 



are in position. 



1. Cranial Bones. — 

 The occipital bone forms 

 the most posterior part 



of the skull, and origin- pharyngeal nerves ; C.c carotid canal; 

 ' o T., tympanic cavity; V., foramen for 



trigeminal nerve; A.t., auditive tube; 

 Q. , quadrate ; Pt. , pterygoid ; Q.J. , quad- 

 rato-jugal; J., jugal ; M., maxilla; PI., 

 palatine; F., frontal; L., lachrymal; 

 X., nasal ; I., incisive. 



Fig. 4.— Base of the SkuU. 



F.m., foramen magnum; O.c, occi- 

 pital condyle; 0., occipital; B.t., basi- 

 temporal ; XII. , foramen for hypoglossal 

 nerve ; X. , foramen for vagus and glosso- 



ally consists of four 

 parts — a basilar, two 

 lateral, and a squamous 

 — grouped around a large opening, the foramen 

 magnum, by which the cavity of the cranium 

 is placed in communication with the vertebral 

 canal. Immediately below the foramen is a 

 single, rounded condyle for articulation with the 



