to a bone known to be homologous to the quadrate of lower 

 vertebrates, indicates the extent of nomenclature difficulties. 

 The term incus is so well established that it seems neither 

 possible nor useful to supplant it. Use of the term malleus 

 is desirable, for this is sometimes a compound bone and, 

 further, it is established and it designates a new functional 

 unit in the mammal. Several compound bones of the 

 human — the occipital (formed by the basioccipital, e.xocci- 

 pital, supraoccipital, and usually the postparietal), the 

 temporal (formed from the squamosal, petrosal, tympanic, 

 and entotympanic when it occurs), and sphenoid (formed 

 from orbito, ali, and basisphenoids, and pterygoid) — are 

 broken down into their components, whereas the petrosal 

 (formed of prootic and opisthotic) is retained. The use of 

 petrosal is justified because anatomists are unable to associ- 

 ate clearly the several ossification centers with the original 

 opisthotic and prootic bones. The styloid process of the 

 petrosal presents a special problem since it has two centers 

 of ossification, an upper tympanohyal and a ventral 

 stylohyal. 



Other terminological problems are related to the follow- 

 ing anatomical facts: the alisphenoid, although basically a 

 chondral bone, has a dermal extension; the pterygoid, 

 although basically a dermal bone, has a chondral part, the 

 hamulus; and the entotympanic, when present, arises from 

 an extracranial cartilage, which, like the cartilage for the 

 hamulus, is not present in lower forms. A more detailed 

 review of the mammalian head skeleton will reveal addi- 

 tional problems and contribute to an understanding of the 

 above points. 



Head skeleton of other placentals The cat, dog, and rabbit 

 are representatives of the placental mammals. These three 

 animals show the differences that exist between two carni- 

 vores, the cat and dog, and between the carnivores and 

 another highly specialized order, the Lagomorpha, the 

 rabbit. 



At first glance the skulls of these mammals present sev- 

 eral main features (Figures 3-2, 3-3). Posteriorly there is 

 the cranial capsule enclosing the brain. From this capsule 

 the foramen magnum opens posteriorly, posteroventrally, or 

 ventrally as in the human. Below the cranial capsule are the 

 bilateral tympanic bullae. Each of these has a tympanic 

 fenestra, or aperture, which in life is spanned by the tympanic 

 membrane, or eardrum. Anterior to the swelling of the cra- 

 nial capsule is the large eye socket on either side, called the 

 orbit. Anterior to the orbit is the snout or muzzle, projecting 

 forward to the anterior narial opening. The mandible or 

 lower jaw articulates with the skull lateral to the cranial cap- 

 sule. The hyoid apparatus, which may or may not be well 

 developed, is suspended from the skull behind the mandib- 

 ular articulation. The rabbit differs in having the surface of 

 many of the bones fenestrated, particularly the ma.xilla in 

 front of the orbit. 



In adult mammals the four occipital bones are fused 



usually into a single piece enclosing the foramen magnum, 

 the opening for the passage of the spinal cord from the cra- 

 nial cavity. The supraoccipital is separate in the rabbit. The 

 postparietal arises in development as a pair of centers that 

 soon fuse at the midline to form a single triangular piece with 

 an apex pointed anteriorly. Soon after its appearance this 

 midline bone usually fuses with the supraoccipital, sometimes 

 with the parietals. In some cats and dogs it remains free. In 

 the rabbit it is a separate ovoid midline bone. 



The exoccipital bones largely bear the bilateral occipital 

 condyles which articulate with the vertebral column. Above 

 the foramen magnum the supraoccipital and exoccipitals are 

 drawn out to form the transverse, crescentic, nuchal 

 (lamboidal) crest — the postparietal lies anterior to the 

 nuchal crest and forms part of the anteroposteriorly oriented 

 sagittal crest in the cat and dog. These crests are related to 

 muscle insertions and origins. 



The basioccipital is a midline bone bearing a part of each 

 occipital condyle on its posterolateral margins. To either 

 side of it are the large swollen tympanic bullae, whose an- 

 terior ends lie opposite the ends of the transverse suture 

 separating the basioccipital and basisphenoid. 



The tympanic bulla, housing the middle-ear space, is a 

 compound structure formed from several sources. In man 

 or the rabbit it involves the tympanic bone and an outgrowth 

 from the petrosal. In some insectivores the basisphenoid 

 contributes a margining process, which, together with the 

 tympanic and the petrosal, forms an incomplete bulla. In 

 the cat and dog the petrosal component is replaced by the 

 entotympanic, which, along with the tympanic, forms a 

 complete bulla. 



The exoccipital has a distinct process extending down 

 behind the bulla. At the posterior inner margin of the bulla 

 there is a large foramen or fossa. This is the posterior lacerate 

 fossa, and the foramen that opens directly through it is the 

 jugular foramen. 



Returning to the top of the skull, we find the paired parietals 

 lie anterior and lateral to the midline postparietal. The parie- 

 tal has distinct sutures; it sutures laterally with the squamosal, 

 anteriorly with the frontal, and, between these, with the 

 alisphenoid (not in the rabbit). 



The squamosal forms the lateral wall of the brain case 

 and from it a large zygomatic process extends outward and 

 forward to contact the jugal. On the under surface at the 

 base of the zygomatic process is the groove of the glenoid 

 articulation. Behind this articular surface is a postglenoid 

 process (lacking in the rabbit). Behind the postglenoid 

 process of the dog, in the crevasse above the tympanic 

 fenestra, is a large postglenoid foramen. A minute foramen 

 occurs in this general region in the cat and rabbit, but the 

 exact position is not comparable. The sutures of the squa- 

 mosal are clear, except in the area around the tympanic 

 fenestra. This bone is fused with the tympanic in the cat or 

 dog, at the upper margin of the tympanic fenestra; posterior 

 to this opening the squamosal is fused with the petrosal. In 



MAMMALIAN HEAD SKELETON 



37 



