INTRODUCTION. xix 



understood without a study of the operations of cell-division, self-fission and gemma- 

 tion in the one-celled animals and polyps. Our most eminent human physiologists, 

 such as Eemak, Bischoff, von Baer, and others, have had to go to the lower animals for 

 facts to illustrate the reproductive processes in man. No medical student can in these 

 days afford to be ignorant of the general laws of animal physiology. 



Locomotion. 



All the movements of the body, or of the internal organs, with which physiology 

 has to do, depend primarily on the contractility inherent in the protoplasm filling 

 the cells of the body. Of the cause of contractility in protoplasm we know nothing. 

 We see its manifestations in the irritability and resulting contractions of the body of 

 the Amoeba, of the white blood-corpuscle, and other cells and one-celled organisms. 

 It is this inherent contractility of the protoplasm in muscle-cells which gives rise to 

 muscular movements. 



But the simplest one-celled animals do not move about by means of muscles. The 

 Amoeba changes its position, Proteus-like, by variously contracting the body, and thus 

 changing its form, throwing out root-like processes in different directions. The 

 Infusoria have permanent thread-like processes, called cilia, by which they can swim 

 about in the water. In the Hydra and other polyps, however, we meet with muscles, 

 by which the body can contract in certain parts ; in such animals the base of the body 

 forms a more or less contractile, movable, creeping disc, while the tentacles move 

 partly by means of their muscular walls, and partly mechanically by filing them with 

 the circulatory fluid of the body. 



The searurchin and starfish move slowly over the sea weed and rocks by means 

 of long, slender suckers. Extending these by allowing the water to flow into them, 

 and fastening them to the surface of the object over which they are moving, they then 

 contract them, and in this way the body is warped slowly along. 



In the lower worms, such as the flat-worms, or in the snails, the gliding movement is 

 due to the muscular contractions of the under side of the body. The gliding motion 

 of snails is due to a system of extensive muscular fibres within the disc, which act 

 when the sinuses within the disc are filled with blood; their extension causing the 

 undulating appearance on the under side of the snail's foot, or creeping disc ; but 

 the snail can only thus move forwards; the lateral movements and shortening of 

 the foot being produced by oblique muscular fibres. 



Rising in the zoological scale, we come to the Crustacea and insects which have 

 jointed legs, ending, in the latter, in claws adapting the limb both for walking and 

 climbing. The legs of arthropods are perhaps modifications of the lateral fleshy 

 oar-like appendages of the sea-worms, which have become externally hard and jointed, 

 with several leverage-systems. The mechanism of locomotion is fundamentally the 

 same in the legs of arthropods an^ vertebrates. Space does not permit us to discuss 

 the subject of the mechanism of walking, running, and flying, but all these movements 

 are dependent primarily on the contractility inherent in the protoplasm filling the cells 

 forming muscular tissue. 



Digestion'. 

 The most important organs in the animal system are those relating to digestion, as 

 an animal may respire solely through its body-walls, or do without a circulatory or 

 nervous system, but must eat in order to live and grow. The opening by which the 



