16 THE ANIMAL KINGDOM. 



ITHE ANIMAL KINGDOM. 



ACCORDING to Cuvier there are four principal forms after 

 which all living beings seem to have been modelled. The basis 

 of these distinctions is laid in the organization of the creatures 

 themselves. Sensation and movement are the characteristics of 

 animals. The heart and the organs of circulation seem a kind 

 of centre for those functions which may be called vegetative, while 

 the brain and the nervous system form the principal source of 

 the functions more exclusively animal. Descending from the 

 higher to the lower races of animals, both these systems are found 

 gradually to become more imperfect, and finally to disappear al- 

 together. In the lowest tribes in the scale, where nerves are no 

 longer visible, the muscular fibre also ceases to be distinct, and 

 the organs of digestion are reduced to a simple cavity in the 

 homogeneous mass. In insects the vascular system disappears 

 even before the nervous system ; but in general the dispersion 

 of the medullary masses is connected with the agents of muscu- 

 lar motion : a spinal marrow upon which knots or ganglia re- 

 present as many brains or seats of sensation, corresponding to 

 the structure of a body divided into numerous rings, and sup- 

 ported by pairs of limbs distributed along these annulations. 

 This relative proportion in the structure of general forms, which 

 results from the arrangement of the organs of motion, from the 

 distribution of the nervous masses, and from the energy of the 

 circulating system, constitutes the basis upon which M. Cuvier 

 has founded the principal divisions of the Animal Kingdom. 



In the first of these general forms, which is that of Man 

 and the animals which resemble him most nearly, the brain and 

 the principal trunk of the nervous system are inclosed in bony 

 cases, the first called the cranium, the second the vertebrae. To 

 the sides of the vertebral column, as to a centre, are attached 

 the ribs, and the bones of the members which form the frame- 

 work of the body. The muscles in general cover the bones, 

 which they put into action, and the viscera are inclosed in the 



