290 COMPARATIVE ANATOMY 



the articulating surfaces of the centra are nearly flat, and that 

 they are united to one another by a tough fibrous and elastic 

 material forming the intervertebral discs. The notochord 

 disappears entirely within the vertebral bodies, but remnants 

 of it may be found in the intervertebral discs forming the so- 

 called nucleus pulposus. 



The dog's skull, of which a full description is given in 

 " Flower's Osteology of the Mammalia," is the most convenient 

 example for practical study. 



The following characteristic points should be specially 

 noted. 



The floor, roof, and walls of the cranium are completely 

 ossified, and the bones connecting them are united by sutures 

 having serrated edges which dovetail into one another. There 

 is no parasphenoid bone, but the basis cranii is formed by 

 three ossifications in the floor of the primitive cartilaginous 

 cranium — viz. the basi-occipital, the basi-sphenoid, and the 

 pre-sphenoid. The occipital region of the skull is completed by 

 the ex-occipitals at the sides and the supra-occipital above the 

 foramen magnum. There are two occipital condyles formed 

 by the ex-occipitals. The cranial cavity is shut off from the 

 nasal cavities in front by the cribriform plate, a bony parti- 

 tion perforated by numerous holes through which the fibres of 

 the olfactory nerve pass. The cribriform plate is the laterally 

 expanded posterior part of the mesethmoid, a vertical plate 

 of bone and cartilage dividing the right and left nasal chambers 

 from one another. Each nasal passage contains two charac- 

 teristic scroll-like bones, of which the upper and posterior 

 is united with the mesethmoid and called the ethmo-turbinal, 

 the lower and anterior is united with the maxilla and called 

 the maxillo-turbinal bone. These bones are formed by the 

 ossification of the rolls of cartilage over the surface of which 

 the olfactory epithelium is spread. The membrane covering 

 the ethmo-turbinal is supplied by the olfactory nerve, but that 

 covering the maxillo-turbinal is supplied by a branch of the fifth 

 nerve. The bones of the upper jaw, forming the face, are 

 firmly united to one another and to the cranium. There is a 

 hard palate formed by horizontal plates given off from the 

 inner sides of the maxillae and palatines. The pterygoids are 

 very small and confined to the base of the skull. The orbit is 

 bounded below by a zygomatic arch formed by the jugal and 



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