THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 117 



reduction in the membrane bones of the roof of the skull is illustrated in 

 Figure 36, page 106. 



The following description applies to the skull of the cat and rabbit. That 

 of the dog is so similar to that of the cat that the same description applies to both. 

 The cat skull is described and figured in R and J, and D, the rabbit skull in B, 

 and P and H, and the dog .skull in R. The student must learn the bones of the 

 mammalian skull and be able to state which are membrane bones and which 

 cartilage bones. In studying the bones locate carefully the sutures between 

 the bones and by this means determine accurately the extent of each bone. 



1. General regions and features of the skull.— The skull is a hard, bony 

 case, in which the limits of the separate bones are marked by wavy or jagged 

 lines, the sutures. Along these lines the bones dovetail into each other, forming 

 firm, immovable joints. The skull may be divided into an anterior facial portion 

 supporting the nose and eyes and a posterior cranial portion inclosing the brain. 

 At the anterior end of the facial portion are the two nasal openings or anterior 

 nares separated in life by a cartilaginous partition, the anterior part of the 

 septum of the nose. At the side of the facial portion is a large cavity, partially 

 separated by projecting bony processes into two cavities: an anterior, large, 

 nearly circular one, the orbit or orbital fossa, which contains the eye in life; and 

 a posterior one, the temporal fossa, filled in life by muscles. The temporal fossa 

 is very small in the rabbit. The lower boundary of these fossae is formed by a 

 projecting arch of bone, the zygomatic arch, a feature very characteristic of the 

 mammalian skull. In the cat an orbital process extends dorsally from the 

 middle of the zygomatic arch and nearly meets a zygomatic process descending 

 from the roof of the skull. These two processes form the posterior boundary 

 of the orbit. In the rabbit the zygomatic process extends backward and down- 

 ward from the roof of the orbit; in life this process is connected to the zygomatic 

 arch by a ligament, thus marking off a small temporal fossa posterior to the 

 ligament. Dorsal to the orbit is a projecting margin of bone, the supraorbital 

 arch, the posterior end of which projects as the zygomatic process already men- 

 tioned. In the rabbit the anterior end of this arch also bears a projecting 

 process. 



The cranial portion of the skull presents the following features. At the 

 posterior end is the large foramen magnum; on each side of this is a projection, 

 the occipital condyle, which articulates with the atlas. Lateral and slightly 

 anterior to each occipital condyle is a conspicuous hollow expansion, the tympanic 

 bulla, which contains the middle ear. On the posterior surface of each bulla 

 are two processes, an anterior mastoid process and a posterior jugular process. 

 The mastoid process will be seen to be part of a bone, differing in its rough and 

 pitted surface from the other bones of the skull; this bone, the petromastoid 

 bone, contains the internal ear. The tympanic bulla opens laterally by a large 

 opening, the external auditory meatus, which in the rabbit is bounded by a bony 



