866 PHYSIOLOGY. 



striking the sensory skin filaments of the chest and abdomen. After 

 the cord is tied there soon follows an increase of C0 2 in the blood. 

 By its presence the activities of the respiratory center of the medulla 

 are instigated. However., the various centers are but feebly irritable 

 at birth and require somewhat heroic stimulation to bring out their 

 activities. This feebleness accounts for the remarkable vitality of 

 the infant and its intense resistance to asphyxiation. 



Development and Growth. 



When the anabolic and catabolic processes are balanced in adult 

 life, the body remains the same in weight. 



The progressive development of the body in height is made in an 

 uneven manner, depending upon different ages. In the first year the 

 growth is about twenty centimeters, in the second year ten centimeters, 

 third year about seven centimeters, from five to sixteen, about five and 

 one-half centimeters each year. In the twentieth year growth is very 

 slight. 



Dr. H. P. Bowditch has shown that growth is most rapid during 

 the earliest periods of life. During the first twelve years boys are from 

 one to two inches taller than girls of the same age. At about twelve 

 and a half years girls begin to grow faster than boys, and during the 

 fourteenth year are about one inch taller than boys of the same age. 

 At fourteen and a half years of age boys again become taller, girls 

 having at this period very nearly completed their growth, while boys 

 continue to grow rapidly till nineteen years of age. 



On' the contrary, the development in thickness and breadth is 

 slower during the first years than at puberty ; toward the fortieth and 

 fiftieth years it attains its maximum. 



The tissues of the organs may increase in two ways : by increase 

 in volume of existing elements or by the multiplication of new cells. 



Bones present certain physiological properties of great interest, 

 for they grow both in length and thickness. The increase in length is 

 at the end of the bone at the junction of the epiphysis with the 

 diaphysis. The increase in thickness is made by means of the 

 periosteum adding new layers of bone on the surface. 



EVOLUTION.* 



All modern conceptions of the immense multiformity in the 

 animated world are based upon the observed facts of perpetuating 



^Contributed by Dr. P. Fischelis. 



