998 SPECIAL PHYSIOLOGY. 



ends of the nerve are once more united, the medullary sheath of the 

 fibres is reformed, the reproductive process beginning at the cicatrix 

 and extending downwards. In young animals, the medullary substance 

 may be restored before the nerve is united. 



GROWTH. 



The human infant, especially, exhibits an imperfect and feeble con- 

 dition at birth, and many changes, besides mere increase of size, take 

 place in it, before it reaches the conditions of puberty and maturity. 

 At birth, the average weight of the male infant is about Tibs., and of 

 the female infant about Gjlbs. The lengths, in the two sexes, are 

 about 18J inches and 18 inches. The nutritive vegetative functions 

 alone exhibit a special activity, those of animal life proper being com- 

 paratively quiescent. The new-born child takes food, and sleeps ; at 

 first, it passes upwards of twenty hours out of the twenty-four, in a 

 state of slumber ; and during the first year, it requires from twelve to 

 fifteen hours' repose. The respiration, circulation, and development 

 of heat are relatively more active than in the adult ; but the power of 

 resisting cold is feeble, and hence protective clothing is necessary. 



The general growth of the body is at first rapid, but afterwards 

 much more gradual. Half the adult height is reached by about the 

 end of the third year, whilst to attain the remaining half, fifteen or 

 eighteen years more are required. At 20 years of age, a Man is 

 rather more than 3J times his height, and about 20 times his weight, 

 at birth. This growth is not equal in all parts of the body, the lower 

 extremities, which were less developed in the embryo, now becoming 

 proportionally more developed: on the other hand, not only the head 

 but also many internal organs, such as the liver, kidneys, and supra- 

 renal bodies, which are proportionally large at birth, afterwards grow 

 relatively more slowly ; the thymus gland even shrinks. The muscu- 

 lar system and the volitional power which commands it, are simulta- 

 neously developed and strengthened. At the end of the third month, 

 the infant easily supports the weight of its head ; at the fourth month, 

 it is able to sit upright ; at the ninth month, it crawls on the ground; 

 before the end of a year, it can, with assistance, step ; and at various 

 times, from one to two years or more, it can stand, and begin to run 

 alone. At six months, it can lisp, and, before the end of the year, 

 can imitate a few definite articulate sounds of one or two syllables. 

 The senses and the mind are gradually brought into exercise, hearing, 

 as indicated by the effect of noises, before sight, as shown by the at- 

 tractiveness of light- or of bright-colored objects. The development 

 of sight, as a source of definite knowledge, under the education of 

 touch, has been already fully explained. Of the other senses, per- 

 haps, taste is the next to be developed, and after that, smell and touch. 

 The order of appearance of the milk and permanent. teeth, has already 

 been detailed. The food of the infant, before it acquires teeth, is 

 fluid, and the entrance of this into the stomach distends that organ, 

 and completes its transverse direction ; after the teeth appear, the 



