I STRUCTURE AND PHYSIOLOGY OF ANIMALS 37 



on. These chemical changes in the tissues, having for their result 

 the production of heat, motion, secretion, and nerve-action, are 

 for the most part of the nature of oxidations, and involve a constant 

 consumption of oxygen ; while a product which becomes formed 

 as a result of this action is carbonic acid gas. 



To carry out all the functions which it has to perform as a 

 distributor of nourishment and oxygen and a remover of carbonic 

 acid, the blood has to be moved about through the vessels to 

 circulate throughout the various organs. In the lowest forms in 

 which a definite blood-system is to be recognised, this movement 

 is effected in great measure by the general movements of the 

 body of the animal. In others certain of the vessels contract and 

 drive the blood through the system ; such contractions are of a 

 peristaltic character, the contractions being of the nature of con- 

 strictions running in a definite direction along the course of the 

 vessel, with an effect similar to that produced by drawing the 

 hand along a compressible india-rubber tube. 



In all higher forms the movement of the blood is effected by 

 means of a special organ the heart. The heart is a muscular 

 organ which by its contractions forces the blood through the 

 system of vessels. In its simplest form it usually consists of two 

 chambers, both with muscular walls, the one, called the auricle, 

 receiving the blood and driving it into the other, which is called 

 the ventricle. The latter, in turn, when it contracts, drives the blood 

 through the vessels to the various parts of the body the return 

 of the blood backwards to the auricle from the ventricle being 

 prevented by the presence of certain valves, which act like folding 

 doors opening from the auricle towards the ventricle, but closing 

 when pressure is exerted in the opposite direction. In the higher 

 animals the heart becomes a more complex organ than this, with a 

 larger number of chambers and a more elaborate system of valves. 



Carbonic acid, as already mentioned, is a waste-product con- 

 stantly being produced in the tissues and being carried off by the 

 blood to pass out by the gills or lungs. Besides the carbonic 

 acid, there are constantly being formed waste-substances of another 

 class viz., substances containing nitrogen, of which uric acid and 

 urea are the principal ultimate forms. These are separated from 

 the blood and thrown out of the body by a distinct set of organs 

 called renal organs, or organs of urinary excretion. The 

 form of these organs varies greatly in the different groups ; 

 in many cases they are more or less intimately connected with 

 the genital system. 



In place of the simple contractions and extensions of the proto- 

 plasm which constitute the only movements of Amoeba, the higher 

 animals are capable of complex and definite movements. These 

 are brought about by the agency of a set of organs termed the 

 muscles. A muscle is a band or sheet of muscular fibres 



