THE SKELETAL SYSTEM 9 



the occipital, interparietal, parietals, frontals, nasals, pre- 

 maxillaries, maxillaries, zygomatics, squamosals, and lac- 

 rimals. The position and structure of each of these bones 

 will be described in detail later. 



Exercise I. Draw and label the dorsal view of the skull. 



Lateral aspect of the cranium. The nuchal plate bears 

 ventrally the occipital condyle, by which the skull articu- 

 lates with the backbone. The jugular process is a long, 

 pointed, triangular structure lateral to and slightly in 

 front of the occipital condyle. Between the two is a semi- 

 circular depression, the jugular fossfl. The mastoid portion 

 of the petrosal l>one is immediately anterior to the jugular 

 process, and is located in a pronounced depression of the 

 lateral part of the occipital bone. The tympanic bulla is a 

 hollow spheroidal capsule anterior to the jugular process, 

 opening to the outside through the external acousiic 

 meatus. This opening is closed in life by the tympanic 

 memhrane, which transmits sound waves from the outer ear 

 to the three small bones of the middle ear. 



Where the mastoid portion connects anteriorly with the 

 squamosal bone there is a ventral extension of the latter, 

 the post-tympanic hook, which helps to hold the tympanic 

 bulla in place. 



The orbito-temporal fossa and zygomatic arch are seen 

 from the lateral side also. Within the fossa are foramina 

 through which the nerves emerge from the brain. The 

 foramen ovale is located directly ventral to the posterior 

 end of the zygomatic arch, and anterior to the tympanic 

 bulla. A branch of the fifth cranial nerve emerges here. 

 The foramen communicates by a perforation of the exter- 

 nal pterygoid process with the ventral surface of the skull. 

 Another small foramen is immediately in front of the fora- 

 men ovale. A few millimeters anterior to the foramen 



