The Skeletal System 103 



lacrimal bone. This is a small bone containing an opening through which 

 tears pass into the nasal cavity. Ventrally the skull consists of a portion 

 of the maxilla, the palatine, the sphenoidal complex, and part of the 

 occipital complex. The hard palate which provides the ventral bound- 

 ary of the air passageway is formed from portions of the maxilla and 

 the palatine. Above the hard palate, the ethnoids and vomers separate 

 the nasal passageways within which are the coiled turbinates. 



The jaws which originate as portions of the visceral skeleton be- 

 come part of the skull in the adult. Both structurally and functionally 

 they cannot be separated from this portion of the axial skeleton even 

 though their origin is somewhat different. The upper jaw is formed 

 principally of portions of the premaxilla and maxilla. The lower jaw 

 or mandible consists of a pair of bones which are fused in front at 

 the mandibular symphysis. Posteriorly the mandible articulates with 

 the mandibular fossa of the temporal bone. 



The cranium is a hollow box of very light though strong con- 

 struction for housing the brain. The cranial cavity is variously shaped 

 and molded to conform to the surface configurations of the brain. At 

 the base of the cranial cavity a small depression, the sella turcica, con- 

 tains the pituitary gland. (Fig. 44.) 



Many protuberances and processes are present on the various bones 

 of the skull. Located on the temporal bone is the rounded tympanic 

 bulla which contains the bones of the ear. Near this are two other 

 projections, the jugal and mastoid processes. At the base of the 

 occipital bone are the two rounded occipital condyles which articulate 

 the skull with the first neck vertebra. The heavy lambdoidal ridge is 

 located at the upper part of the occipital bone. This latter ridge and 

 the jugal and mastoid processes are important for muscle attachments. 

 They are but a few of the many processes present on the skull. 



There are approximately twenty-three openings present on por- 

 tions of the mammalian skull for the passage of nerves and blood 

 vessels. The largest of these is at the base of the occipital bone 

 between the occipital condyles. This is the foramen magnum through 

 which the posterior portion of the brain passes to become the spinal 

 cord. At the back of the orbit of the eye is a foramen for the passage 

 of the optic nerve. The large infraorbital foramen in the maxilla 

 permits the passage of an important nerv^e, while the external audi- 

 tory meatus in the tympanic bulla provides an inlet for the ear. 



The frontal, ethnoid, maxillary, and sphenoidals have spaces or cav- 

 ities within them known as sinuses (Fig. 44). These sinuses connect 



