THE ANIMAL KINGDOM. 



ture be merely such a rod, or whether it be divided into the numerous bony 

 segments known as vertebrae, it is invariably situated on the dorsal aspect of 

 the body, or that from which the limbs are turned away. On that side of 

 the primitive backbone lying nearest to the back, there runs a tube or canal, 

 formed by arches of bone or cartilage arising from the bodies of the vertebrce, 

 and containinc? the great nervous cord commonly spoken of as the spinal 

 marrow or cord. This cord in the adult state of the higher forms being thus 

 included within what are called the arches of the vertebrae. On the side of 

 the primitive backbone opposite to that occupied by the spinal marrow is a 

 much lar^r chamber, containing the heart and digestive organs ; so that, in 

 cross-section, the body of a Vertebrate consists of a smaller nerve-tube lying 

 above the primitive backbone, and of a much larger tube, containing the 

 heart and digestive organs below the latter. Throughout the Vertebrates the 

 limbs never exceed two pairs, and are always turned away from that aspect 

 of the body containing the nerve-tubo ; and^ the two jaws are upper and 

 lower, and work against one another in a vertical plane. 



In addition to the presence of milk -glands, and the suckling by the young 



of the fluid they secrete, the following structural features may be noticed as 



distinctive of the Mammalian class. A highly important 



Distmctive feature is the mode of articulation of the lower jaw to the 



K^mmals" ^"^ proper, or cranium ; this being effected by means of a 

 prominence, or condyle, at the higher portion of the hinder ex- 

 tremity of the lower jaw, articulating with a special cavity — the glenoid cavity-- 

 in the cranium. On the other hand, in the lower Vertebrates this articulation is 

 effected by means of. a special separate bone — the quadrate — articulating above 

 with the cranium, and below with the lower jaw ; this quadrate, as such, 

 being absent in the Mammalia. Another peculiarity connected with the 

 lower jaw is that it consists of a right and left branch — connected at the chin 

 by a bony or cartilaginous union — each of which is formed of but a single bone; 

 whereas in the inferior Vertebrates several distinct bones enter into the com- 

 position of the two branches. Externally, Mammals are further characterised 

 by the possession of hair on the skin, although this may be represented 

 merely by a few bristles in the neighbourhood of the mouth during the earlier 

 stages of existence. Internally, that portion of the great body-cavity con- 

 taining the heart and lungs is completely shut off from the chamber in which 

 are placed the digestive organs by means of a transverse partition, known as 

 the midriff or diaphragm. Existing Mammals, at any rate, have also a higher 

 type of brain than the inferior classes, an especial feature being the presence 

 of a transverse band of tissue on its lower surface, connecting together the 

 main lateral halves or hemispheres. 



Such are some of the leading characters by which the Mammalia may be 

 distinguished from the whole of the other Vertebrates. From both Birds 

 and Reptiles the class differs by the circumstance that the skull is movably 

 articulated to the first vertebra of the neck by means of two knobs, or con- 

 dyles, situated one on each side of the lower part of the aperture, or foramen, 

 through which the spinal cord passes into the skull to join the brain. Two 

 condyles are, however, present in the Amphibians (frogs and salamanders), 

 which differ from Mammals in their cold blood. A second distinction from 

 Birds and Reptiles is to be found in the structure of the ankle joint ; the 

 movable joint in Mammals being situated above the ankle, whereas in the 

 other groups it is placed in the middle of the same. In common with Birds, 

 Mammals differ from the three lower classes of the Vertebrata in havintr 



