from the posterior cerebellar fossa by the tentorium, a 



secondary ossification in the connective tissue between these 

 two parts of the brain. The tentorium is incomplete dorsally 

 in the rabbit, incomplete laterally in the dog. The inner 

 walls of the cranium are irregularly molded, following the 

 surface of the brain and its investing connective tissue and 

 blood vessels. The sella turcica, into which the pituitary 

 gland fits, is a shallow depression in front of a transverse 

 raised dorsum sellae. The margin of the dorsum sellae is 

 separated by a small gap form the tentorium in the dog, a 

 larger gap in the cat. The dorsolateral margin of the dorsum 

 sellae extends forward as a posterior clinoid process. An 

 anterior clinoid process is indicated on the orbitosphenoid 

 anterior to the tip of the posterior process. 



In the lateral wall of the cerebellar fossa is the main part 

 of the petrosal. The surface of this somewhat triangular 

 mass, whose apex points anteromedially, is marked by two 

 large pits; these are the internal acoustic meatus below and 

 the subarcuate or floccular fossa above. 



The mandible (Figure 3-2 A) is composed of two rami 

 joined together anteriorly at a symphysis. Each mandibular 

 ramus is composed entirely of the dentary bone. This is 

 drawn up posteriorly into a tall coronoid process extending 

 upward inside the zygomatic process of the skull and filling 

 much of the temporal fossa. In the rabbit such a process is 

 lacking; the anterior margin of the articular process, which 

 resembles the coronoid process of the cat or dog, is grooved, 

 and its lateral margin may be drawn out as a slight coronoid 

 process. 



Behind the coronoid process of the dog or cat is the arti- 

 cular process bearing a transverse articulatory condyle. In 

 the rabbit the condyle is small and is found at the anterior 

 margin of the large dorsally extended articular process. 

 Below the articular process is a strong angular process. 



In their dentition of the lower jaw, the dog and cat are 

 similar, except that the dog has more teeth in its longer 

 ramus; none of these teeth is molariform (shaped like a 

 molar). What is designated as the first molar is also the lower 

 carnassial. The rabbit has only one incisor, no canine, two 

 premolars, and three molars in the lower jaw. The premolars 

 and molars are all similar high-crowned, continuously grow- 

 ing, peg-like teeth with transverse lophs or crests. 



Several bones of the reptile lower jaw and visceral skeleton 

 are involved in the mammalian middle-ear structure (see 

 Chapter 4). There are three ossicles for conducting vibrations 

 from the tympanum to the inner ear (Figure 3-4): the malleus, 

 incus, and stapes. The last of these belongs to the hyoid arch. 

 Its foot plate fits into the fenestra vestibuli of the petrosal. 

 The incus is the homolog of the reptilian quadrate which 

 articulates between the cranium and the lower jaw. The 

 malleus is a compound bone formed by fusion of two lower 

 jaw bones, the prearticular, which forms the anterior proc- 

 ess of the malleus, and the articular. In addition to the 

 malleus, the tympanic bone (see above) is of lower jaw ori- 

 gin. It represents the reptilian angular. 



The hyoid apparatus of the cat consists of four bony seg- 

 ments extending down from the region of the mastoid proc- 

 ess of the petrosal to a transverse body. These are, from 

 above downward, the stylohyal, epihyal, ceratohyal, and 

 hypohal. From the body a large ceratobranchial I (thyro- 

 hyalj extends back and upward to the thyroid cartilage. In 

 the dog there are three segments in the hyoid arch; from 

 the body upward these are the hypohyal, ceratohyal, and 

 epihyal. The stylohyoid ligament connects these bones with 

 the bulla. In the rabbit the stylohyal is anchored by con- 

 nective tissue to the bulla and the exoccipital process. It 

 serves as origin for the styloglossal muscle and for attach- 

 ment of the stylohyoid ligament which extends down to the 

 small bony hypohyal. The body of the hyoid is large and 

 there is a well-developed ceratobranchial I (greater or major 

 cornua). In mammals generally, there is an ossified process 

 for the attachment of the hyoid arch to the cranium. This 

 process is the tympanohyal process. It is concealed within 

 the bulla and thus not readily observable. 



Joints of the head skeleton The joints, or articulations, 

 between the bones of the mammalian head skeleton can 

 now be considered in terms of their variations. These joints 

 are for the most part immovable, some are permissive of 

 slight flexion, others articulate freely. The immovable are 

 called synarthroses (singular synarthrosis), the others diar- 

 throses. Examples of the middle group are considered as 

 synarthroses or amphiarthroses. The extreme of the synar- 

 throsis is the synostosis, where the two bones are fused to- 

 gether, sometimes indistinguishably. In a synarthrosis the 

 edges of the bones may be separated by little or much 

 connective tissue. 



There are several kinds of synarthroses in head skeletons. 

 These include: sutures (true and false), schindyleses, and 

 gomphoses. True sutures are joints formed by interlocking 

 finger-like (sutura dentata) or tooth-like (sutura serrata) 

 projections of the bone margins. More complex joints in- 

 volve both interlocking and overlapping of edges, the sutura 

 limbosa. False sutures are those with opposing roughened 

 edges, called the harmonious suture, or with overlapping 

 bones, called the squamose suture. The harmonious suture 

 has the extreme of perfectly plane surfaces appressed together 

 as in the case of the midline suture of the nasals. 



In some joints a plate of one bone lies between laminae 

 of another or two other bones; this is a schindylesis. An 

 example of this would be the joint between the perpendi- 

 cular plate of the ethmoid and the vomers. 



Gomphosis is the term for the joint between the peg- 

 like roots of the teeth and the walls of the sockets into which 

 they fit. 



Many of the skull joints undergo fusion during the devel- 

 opmental process and disappear (by synostosis). In some 

 cases the line of the old joint may remain; in other cases 

 even sections through the bone do not reveal the line of 

 juncture. In the skulls of young animals, bones of the endo- 



MAMMALIAN HEAD SKELETON 



41 



