24 INTRODUCTION. 



Thus nerves are the messengers by which the mind receives 

 information of the external world (nuntii rerum}. 



But the nerves are equally the ministers of the will, which by 

 their assistance is able to act upon the muscles. By Muscles are 

 understood those active organs of motion (organa motus activa) 

 which are fixed to other parts, as their point of resistance, and 

 these last are called passive organs of motion (organa motus passiva], 

 The harder fibres, which serve for the insertion of muscles, form 

 Tendons, of which the colour in animals with red flesh, as in man, 

 is white. In many animals the muscles are inserted into the skin, 

 or into certain hard portions of the skin, as in Insects, whose hard 

 and often horny coverings supply the place of a skeleton in that 

 respect. A skeleton is, properly, a connected whole of internal 

 passive organs of motion cartilaginous or bony, and these serve 

 not only for motion, but moreover, and indeed especially, for the 

 protection of the most important parts of the nervous system, the 

 Brain and Spinal Cord. The skull (for the protection of the Brain) 

 and the Vertebral Column (which encloses the Spinal Cord) must 

 therefore be considered as the principal parts of the skeleton, of 

 which ribs and limbs are only appendages: in this simple condi- 

 tion is the skeleton met with in the Larva, for example, of the 

 Frog. 



Development of Animals. 



How ike expression imperfect Animal is to be understood. 



We have attempted to give a general idea of the organs which 

 compose the animal body. But these organs are by no means 

 found in all animals. Only in the more perfect animals is the 

 structure thus complicated. When from these we descend in the 

 animal scale, we perceive in the long series one instrument after 

 another gradually decrease in magnitude and development, and at 

 last entirely disappear. In Polyps (hydras) nothing remains but 

 the Intestinal Canal. The entire animal forms a blind sac com- 

 posed of a single tissue, and all the vital functions which it 

 performs are effected through one and the same gelatinous mass. 

 Finally, in some Infusories we no longer perceive even an intestinal 

 canal nothing remains but an homogeneous gelatinous body, whose 



