278 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Skull Of the Dog. This agrees in all essential matters with the rabbit's 

 skull, but there are differences in general shape and also in certain details. 

 Most of these variations are correlated with the larger development of the 

 jaw-muscles, as may be seen, for instance, in the presence of strong ridges 

 on the brain-case and the width of the zygomatic arches. The skull is much 

 stronger, and the face is in line with the cranium, whereas in the rabbit it 

 is bent slightly downwards. The spongy areas present in the skull of the 

 rabbit (especially on the outer side of the maxilla) are absent in the dog. 

 Other differences are the following: (1) Cranium. Plane of occipital 

 region vertical, instead of slanting somewhat downwards ; interparietal 

 represented by a process of supra-occipital. (2) Olfactory capsules. Naso- 

 turbinal represented by uppermost lamella of ethmo-turbinal. (3) Bony 

 palate very large and long. (4) Mandible, with short ascending part, well- 

 marked angle, large coronoid, and transverse condyle, articulating with 

 glenoid fossa to form a well-marked hinge-joint. All these last features 

 are connected with the great size of the muscles used in biting. The hinge- 

 joint only allows of snapping movements, giving no lateral play. (5) Nerve- 

 exits. The 2nd and 3rd branches of the trigeminal exit by special openings, 

 foramen rotundum and foramen ovale, situated one behind the other in the 

 ali-sphenoid, between the sphenoidal fissure and the foramen lacerum 

 medium. (6) Teeth, see p. 285. 



(b) Vertebral Column and Ribs (Fig. 84). The vertebral 

 column is divisible into cervical, thoracic, lumbar, sacral, and 

 caudal regions, corresponding to neck, thorax, loins, pelvis, and 

 tail. Except in the sacral region the vertebrae remain distinct 

 from one another, and the more or less flat ends of the centra 

 are connected together by cartilaginous discs, intervertebra3, 

 which come between them. The front and back of each centrum 

 ossify independently as thin bony plates, epiphyses, which fuse 

 later on with the middle part. The spinal nerves exit as in the 

 other types by intervertebral foramina between adjoining arches, 

 and neural spines and zygapophyses of the usual kind are present. 



The cervical vertebrae are seven in number. Their centra are 

 short, and their neural spines small. The cervical ribs are not 

 free but fused, as in most of the pigeon's cervical vertebrae 

 (cf. p. 237). The apparent transverse processes are thus perforated 

 at their bases to form the vertebrarterial canal, and are divided 

 into two parts, the upper of which is true transverse process, the 

 lower (inferior lamina} the projecting part of the fused rib. 



The atlas or first cervical vertebra is ring-like, possesses large 

 transverse processes (but no inferior laminae), and a thin narrow 

 centrum. It presents in front two large oval articular surfaces 

 for the occipital condyles, and behind two smaller concavities for 

 the axis, but there are no true zygapophyses. 



