VARIABILITY 25 



although the thoracic portion reaches its maximum in the earher stages, and the pelvic portion 

 not until adult hfe. The lower limbs, hke the pelvis, develop slowly, forming about 20 per cent, 

 of the body at birth and reaching 35 per cent, in the adult. 



Relative growth of the systems. — There is also a marked difference in the relative growth 

 of the various systems. Data for the skin and skeleton are somewhat scanty and unsatis- 

 factory. The musculature, however, is relatively small in the embryo, increasing to about 25 

 per cent, of the body in the newborn, and to 40 or 45 per cent, in the adult. The visceral group 

 (including brain and spinal cord), on the other hand, is relatively largest in the early embryo, 

 decreasing from about 35 per cent, of the body to about 24 per cent, in the newborn and to about 

 10 per cent, in the adult. 



Relative growth of the organs. — While in general, the individual organs follow the course 

 of relative growth of the visceral group, each organ has its own characteristic course of growth. 

 As a rule, after its appearance in the embryo, each organ increases more or less rapidly to its 

 maximum relative size, after which, although increasing in absolute size, it decreases in relative 

 size through subsequent prenatal and postnatal life up to the adult. 



Thus the brain in the embryo of the second month forms more than 20 per cent, of the body, 

 but steadily declines to about 13 or 14 per cent, in the newborn, and about 2 per cent, in the 

 adult. The spinal cord and eyeballs have a similar course of growth. The heart declines from 

 about 5 per cent, of the body in the embryo of the second month to about .75 per cent, in the 

 newborn and .46 per cent, in the adult. The liver decreases from a maximum of nearly 10 per 

 cent, in the third month to 5 per cent, in the newborn and 2.7 per cent in the adult. The supra- 

 renal glands decrea.se from about .46 per cent, of the body in the third month to .23 per cent, 

 in the newborn and .01 per cent, in the adult. The lungs decrease from 3.3 per cent, in the 

 fourth month to about 2 per cent, of the body at birth and 1 per cent, (bloodless weight) in 

 the adult._ The kidneys reach a maximum of about 1 per cent, of the body toward the end of the 

 foetal period, decreasing to about .46 per cent, in the adult. The thymus, thyroid, spleen and 

 alimentary canal Ukewise reach their maximum slowly, being probably relatively largest about 

 the time of birth. The ovary and testis, however, appear to be relatively largest during the 

 prenatal period. 



Variability. — It must be borne in mind that all statements concerning struc- 

 ture refer to the average or norm, and are always subject to variation. This is 

 therefore a topic of importance to students of anatomy. Variations are classified 

 as either germinal or somatic. 



Germinal variations are due to fundamental differences in the germ plasm, and are trans- 

 mitted by heredity. These include many of the characters whereby one individual differs from 

 another. Variations according to sex are included under this class. Variations inherited from 

 more or less remote ancestors are termed atavistic or reversional. 



Somatic variations, or 'acquired characters,' are due to environmental influences, such 

 as nutrition, temperature, shelter, disease, training, etc. While somatic variations may be 

 very great, they do not affect the germ plasm and are not transmitted by heredity. 



In many cases it is exceedingly difficult to distinguish germinal from somatic variations 

 Size, for example, may be due to either or both. Moreover, somatic variations may be pro- 

 duced at any time after the fertilisation of the ovum. Very slight environmental changes are 

 sometimes sufficient to produce a marked effect upon the delicately balanced mechanism of 

 the developing embryo. Malformations and pathological conditions are thus often to be 

 explained. As to the extent of variability, some characters are much more variable than others. 

 Height, for example, is less variable than weight. Moreover, variability differs in the various 

 parts and organs. In general, the head and head organs are less variable than the remainder 

 of the body. The skeleton and musculature appear less variable than the integument and 

 viscera. 



Details concerning variations and methods for their measurement may be found in works 

 on genetics and biometrical statistics. 



References.— E77ibryology: Keibel and Mall, Human Embryology (2 vols.); 

 Bryce, Quain's Anatomy, 11th ed., vol. 1 ; Minot, Laboratory Text-book of Embr}-- 

 ology; McMurrich, Development of the Human Body. Growth: INIinot, Age, 

 Growth, and Death; Jackson, Amer. Jour. Anat., vol. 9; Anat. Record, vol. 3. 

 Heredity: Davenport, Heredity and Eugenics; Walter, Genetics. Biometry: 

 Davenport, Statistical Methods; Yule, Theory of Statistics. 



