120 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



5. The otic capsules. — As already explained, the bones of the otic capsules 

 are all fused together and fused with the squamosal and the tympanic bulla to 

 form the temporal bone, a bone very characteristic of mammals (Fig. 40C, p. 127). 

 That part of the temporal bones which is composed of the otic bones is named the 

 periotic or petromastoid bone. The separate otic bones of which the petromas- 

 toid is composed can be seen only in early embryonic stages. The mastoid portion 

 of this bone is visible on the external surface of the skull between the bulla and 

 the occipital; it projects over the bulla as the mastoid process, very prominent in 

 man as the bump behind the pinna. The petrous portion of the petromastoid bone 

 incloses the internal ear and is visible only from the inside of the skull. The 

 tympanic bulla consists of one or two membrane bones of uncertain homology; 

 it incloses the middle ear and the three middle ear bones. These can be seen in 

 well-cleaned cat skulls as a delicate chain of bones extending across the anterior 

 part of the tympanic cavity. If separate specimens are available, identify 

 them as follows: the malleus or hammer, a slender bone terminating in a knob; 

 the incus or anvil, a smaller bone with two pointed processes; and the stapes or 

 stirrup, shaped like a stirrup. There is good reason to believe that the malleus 

 represents a very much reduced articular; that the incus is the reduced quadrate; 

 and the stapes is the hyomandibular. 



6. The posterior sphenoid region. — On the ventral surface of the skull in 

 front of the ventral part of the occipital bone is the basis phenoid. The basi- 

 sphenoid extends laterally in front of the tympanic bullae as processes, the wings 

 of the basisphenoid. These wings are in reality the alisplienoids and are separate 

 in young stages (see Fig. 40B). The alisphenoids meet the squamous part of 

 the temporal bones dorsally. The alisphenoids extend forward as the pterygoid 

 processes which meet the posterior ends of the palatines. In the rabbit the 

 pterygoid process presents two backwardly projecting thin plates of bone, the 

 lateral and medial lamellae. These inclose between them the pterygoid fossa. 

 The medial lamella in the rabbit and the pterygoid process itself in the cat bear 

 a pointed process, the hamulus. The pterygoid processes correspond to the 

 pterygoid bones of lower forms. 



7. The anterior sphenoid region. — In the median ventral line in front of 

 the basisphenoid is the presphenoid, a slender bone. In the rabbit this is at the 

 bottom of the deep cleft between the palatines. The presphenoid sends up 

 wings into the orbit which meet the frontal bones above and contain the large 

 optic foramen for the passage of the optic nerve. In the cat this foramen is 

 the most anterior of a row of four foramina. To see the wings of the presphenoid 

 turn the skull to obtain a lateral view. The wings are in reality the orbito sphenoid 

 bones of lower vertebrates and are separate in young stages (Fig. 40B, p. 127). 



Draw a ventral view of the skull, showing the sutures accurately. 



8. The ethmoid region and the sagittal section of the skull.— The ethmoid 

 is nearly completely ossified in mammals but can be studied only in sagittal 



