242 HUMAN PHYSIOLOGY. 



Physiologic Activities of the Embryo. During intrauterine life the 

 evolution of structure is accompanied by an evolution of function. The 

 relatively simple and uniform metabolism of the undifferentiated blastodermic 

 membranes gradually increases in complexity and variety, as the individual 

 tissues and organs make their appearance and assume even a slight degree of 

 functional activity. As to the periods at which different organs begin to 

 functionate, but little is positively known. 



The primitive heart, in all probability, begins to pulsate very early, as in an 

 embryo from fifteen to eighteen days old and measuring but 2.2 mm. in length, 

 Coste found the amnion, the allantois, the omphalo-mesenteric vessels, and 

 the two primitive aortae developed. In the earlier weeks, all products of 

 metabolism are doubtless eliminated by the placental structures; but as 

 metabolism increases in complexity the liver and kidney assume excretory 

 activity. Thus, at the end of the third month the intestine contains a dark, 

 greenish, viscid material meconium composed of bile pigments, bile salts, 

 and desquamated epithelium; the ammo tic fluid, as well as the fluid 

 within the bladder, contains urea at the end of the sixth month, indicating the 

 establishment of both hepatic and renal activity. Contractions of the skeletal 

 muscles of the limbs begin about the fifth month, from which it may be 

 inferred that the mechanism for muscle activity, viz., muscles, efferent nerves, 

 and spinal centers, has become anatomically developed and associated, and 

 capable of coordinate activity. These contractions are, in all probability, 

 automatic or autochthonic in character due to stimuli arising within the 

 spinal centers. The remaining organs remain more or less inactive. 



After birth, with the first inspiration and introduction of food into the ali- 

 mentary canal, the physiologic mechanisms which subserve general metab- 

 olism begin to functionate and in the course of a week are fully established. 

 At this time the cardiac pulsation averages about 135 a minute; the respira- 

 tory movements vary from 30 to 35 a minute, and are diaphragmatic in type; 

 the urine, which was at first scanty, is now abundant and proportional to the 

 food consumed; the digestive glands are elaborating their respective enzymes, 

 digestion proceeding as in the adult. The hepatic secretion is active and the 

 lower bowel is emptied of its contents; the coordinate activities of the nerve-, 

 muscle-, and gland-mechanisms are entirely reflex in character. Psychic 

 activities are in abeyance by reason of the incomplete development of the 

 cerebral mechanisms. 



