ij INTRODUCTION 9 



skill. The waste nitrogen, however, and also the waste sulphur 

 (but this is onh^ a small amount, since sulphur is present in only 

 small quant it}' in protoplasm) are got rid of for the most part 

 b}^ the kidneys, which are commonly described as excretory 

 organs. For just as the cells of the body discharge their waste 

 carbon dioxide into the blood, so also do they dispose of their 

 nitrogenous products, and these are carried to the kidneys. In 

 the latter the blood capillaries are separated hj only a single 

 cellular layer from the cavity which communicates with the 

 exterior. The cells of this layer absorb the waste products and 

 excrete them into the kidney tvibules, together with water and 

 some salts in solution. These form the urine, which flo\\s down 

 a duct termed the ureter into the bladder, from which it is ex- 

 pelled at intervals to the exterior in the act of micturition. 



Muscular System. A muscle is an especially contractile 

 organ which is either circular (as in sphincter muscles) or straight. 

 The latter is the more usual form among the higher animals. 

 Such a muscle, upon contracting, reduces the length between its 

 two farthest points. One of these points is usually called the 

 origin of the muscle and the other its insertion. The muscles 

 are attached to bones, and these by their movements may 

 become mclmed to one another at various angles. In the case 

 of the limbs these angles are ojoened and closed, thereby causing 

 progression, and the mechanical aid which is mtroduced to effect 

 this is that of the lever. The muscles themselves are flesh j" 

 masses composed of fibres. Some muscles, such as those of the 

 limbs, are under the direct control of the will and are conse- 

 quently often called voluntary muscles ; whereas others like the 

 muscles surrounding the intestines, which are not under the con- 

 trol of the will, are termed involuntary muscles. The voluntary 

 muscles form a considerable part of the body, and constitute the 

 flesh or meat on an animal. Broadh' speaking, the muscular 

 system is that part of the body in which the energy set free by 

 the oxidation of the food material is converted into motion and 

 so carries on work. 



Nervous System and Sense Organs. In the lo^vest forms of 

 animal life the protoplasm is uniformly irritable and contractile ; 

 but in the higher types, as just shown, the organs of movement 

 are concentrated in a motor or muscular system, ^hile there are 

 also definite organs of sense (eye, ear, etc.). Such division of 



