I] INTRODUCTION 1 1 



is presided over by a special centre in the central nervous system, 

 these centres being the parts of the brain and spinal cord that 

 receive the aflferent impulses and dispatch the efferent ones 

 \\hich are concerned in the reflexes in question. In the body of 

 a higher animal a close succession of reflex actions is continually 

 going on. and it is in this way that the individual organism is 

 able to react to environmental changes, and so fit itself to its 

 surroundings. 



Reproductive System. In the unicellular organisms like the 

 amoeba, reproduction is carried on by simple cell division. In 

 the higher animals, however, there are certain sj^ecial cells set 

 apart for reproduction, the ova in the female and the sperma- 

 tozoa in the male. The ova and spermatozoa are produced 

 respectively in the ovaries and testicles. Their function is to 

 unite together, a single ovum fusing with a single spermatozoon 

 and giving rise to a conjugated cell or oosperm, which by a long 

 succession of cell divisions develops into a new individual. In 

 each of these divisions the cell nuclei also divide so that each 

 product of division always contains a nucleus. In the mammal 

 the development of the unborn J^oung takes place in a si^ecial 

 organ, the uterus, ' womb ' or ' bed ' whence they are expelled 

 in the act of parturition or giving birth. 



Organs of Internal Secretion. In addition to the various 

 systems described above, ^^■e find also certain organs that have 

 the power developed to a special degree of altering the com- 

 position of the blood b}^ secreting into it chemical substances 

 which are elaborated for the advantage of other parts of the 

 body, whither they are conveyed in the blood stream. Many of 

 the organs referred to above which subserve the functions already 

 mentioned are in addition internalh' secreting glands, thus ful- 

 filling more than one purpose. Such organs are the liver, the 

 pancreas, the ovary, and the testicle. The liver, besides secreting 

 bile, stores up a supply of carbohj^drate in the form of gU'cogen, 

 and when required liberates it into the blood as sugar. The 

 panoreas also in some unknown way controls the carbohydrate 

 metabolism, since after its experimental removal sugar makes 

 its appearance in the urine, thereby indicating an excess of sugar 

 in the blood. But there are certain other organs, which appear 

 to be solely organs of internal secretion. Such are the supra- 

 renal bodies, which are situated just in front of the kidneys, one 



