frontal 



condylar foramen 

 A exoccipital 



tympanic 



uamosol 



stapes 

 stapedial muscle 

 ^stylomastoid foramen 



Figure 3-4. Details of tympanic region of human term foetus. B shows outline of skull for orientation 

 of sketch A. 



cranium may be joined by cartilaginous areas. This type of 

 union is identified by the term synchondrosis. Such a union 

 is usually only a developmental condition; in lower verte- 

 brates it frequently occurs in adults. Also during the devel- 

 opmental period dermal bones may be separated by large 

 connective tissue gaps or fontanelles. 



One kind of amphiarthrosis is the symphysis between the 

 rami of the lower jaw in lower forms. In some mammals 



this joint has become a true suture, and in others (man) fus- 

 ion or ankylosis (synostosis) has taken place. Another kind 

 of amphiarthrosis is syndesmosis; that is, many bones are 

 joined by ligaments but do not freely articulate. 



Movable joints, or diarthroses, are found in the head. One 

 example is the articulation of the mandible with the 

 squamosal. Here are two smooth, cartilage-covered artic- 

 ular surfaces, one a ball and the other socket-like; there is 



42 • OSTEOLOGY AND THE MAMMALIAN HEAD SKELETON 



