MAMMALIA 



CLASS V 



and sense-organs are better developed, and their dentition and limbs are more 

 extensively differentiated than in any other class of animals. Only reptiles and 

 fishes are comparable with mammals in the multiplicity of their adaptations. 

 With the single exception of the monotremes the offspring is born alive. The 

 embryos are provided Avith amnion and allantois. For a longer or shorter period 

 the young are nourished on milk, secreted from the mother's mammary glands. 



The vertebral column of mammals is divisible into five regions : cervical, 

 dorsal, lumbar, sacral and caudal (the sacral region being undifterentiated only 

 in the Cetacea and Sirenia). The vertebral centra do not present articular 

 faces to each other as in reptiles and birds, but their plane surfaces are separated 

 by discs or pads of fibro-cartilage ; only the cervicals of cei^tain ungulates depart 

 from this rule, being united by opisthocoelous articulations. Each vertebra con- 

 sists of a middle part, or centrum, to which are applied two thin discs of bone, 

 the epiphyses, one at either end. On the centrum rest the two neural arches 

 which early unite with each other and the centrum, all five elements fusing in 

 the adult into a single bone, the vertebra. 



The neck usually consists of seven vertebrae, the length depending on the 

 elongation of the individual components. The transverse processes are perforated 

 at their bases, forming a canal for the anterior vertebral artery. 



The first cervical, the atlas (Fig. 1), is distinguished by the lack of a centrum 

 and spinous processes, and by the extensive development of the ti^ansverse pro- 

 cesses, the bases of which at the front end are excavated ^^ 

 into two deep articular cups which receive the condyles on 

 the back of the skull. Posteriorly the atlas has a broad 



Fig. 1. 

 First cervical vertebra (atlas) of the 

 dog (dorsal view), d. Expanded trans- 

 verse process ; v, arterial canal ; sn, 

 nerve canal. 



Fifi. 2. 



Second cervical vertebra (axis) of 

 the dog (lateral aspect), sp, Neural 

 spine ; c, centrum ; d, diapophysis ; 

 z, post-zygapophysis ; v, arterial 

 canal ; po, odontoid process ; co, 

 articular surface for the atlas. 



Fig. 3. 



Sixth cervical vertebra of 

 the dog (posterior aspect), c, 

 Centrum ; sp, neural spine ; «., 

 upper arch ; d, diapophysis ; 

 }), parapophysis with inferiorly 

 elongated lamella ; v, arterial 

 canal ; ni, neural canal ; z, 

 anterior ; ^ , posterior zyga- 

 pophysis. 



vertical articular facet for the second cervical or epistropheus (axis). This 

 latter (Fig. 2) is distinguished by having in front a strong, conical, semi- 

 cylindrical, or spout-shaped odontoid process ; Avhich, as shown by its develop- 

 ment, is the centrum of the atlas. 



The dorsal vertebrae (Fig. i. A) are distinguished by having short centra, 

 long spinous processes, and the facets for the heads of the ribs ; one for the 

 tuberculum, on the transverse process, and one for the capitulum, on the antei'ior 



Saugetiere Cliiuas. Abli. k. bayer. Akad. d. Wiss. II. CI., vol. x.xii., 1903.— &o«, W. B., A 

 History of Laud Mammals in the Western Hemisphere. New York, 1913. — Scott, W. B., and 

 Sinclair, W. J., Mammalia of the Santa Cruz Beds. Kept. Princeton University Expedition to 

 Patagonia, 1903-1909. — Weher, M., Die Saugetiere. Jena, 1904. — Wm-tman, J. L., Articles in 

 BulL^Amer. Mus. Nat. Hist. New York, 1892-99, and Amer. Journ. Sci., 1901-1902. 



