Growth Changes of Absolute and Relative Weights of Extremity Muscles of Human Fetus, and Comparison with Adult and Macacus ilus

Abstract

Materials are 138 human fetuses (including from 4 to 10 month), 1 adult male and 27 monkeys (Macacus irus). The muscles are weighed as flexor, extensor, adductor and fibular muscle groups.
Our conclusions are as follows:
(1) Neither side nor sex differences of absolute weights of muscle groups were proved as a whole in fetus and mokey. As for the former it is probably due to the fact that the right handed (or leged) exist neither in fetus nor in monkeys. As for the latter it may be found in absolute weight, but could not be proved at the present stage of study.
In adult male, the right upper limb muscles were apparently heavier than the left, while the left lower limb muscles were heavier than the right except the fibular muscles.
(2) The speed of development of fetal muscles becomes more and more rapid as the fetal age proceeds, and is most remarkable from 9 to 10 month.
(3) As for the order of absolute weights of muscle groups, no change is recognized on lower limb from fetus to adult, but on upper limb there occur great changes; in 4 month arm extensor group is the heaviest, forearm extensor group the lightest and thereafter forearm flexor grows up rapidly, surpasses extensor group after 6 month and, accompanying to it, forearm extensor group grows up remarkably. After 7 month up to 10 month the order of weight of muscle groups is; forearm flexor>arm extensor>forearm extensor>arm flexor.
This change is, we suppose, a preparatory growth for the birth enabling him to suck the milk by gripping his mother's body after birth.
In adult stage, the order of muscles weights is; arm extensor>forearm flexor>arm flexor>forearm extensor.
Developing process of upper limb muscle weight from suckling to adult is not yet cleared because of the difficulty of obtaining materials of these stages.
(4) On lower limb, the order of muscles weights mentioned above remains consistent from fetus to adult: that is; thigh extensor>adductor>leg flexor thigh flexor>leg extensor>fibular.
(5) In the stage of fetus, individual difference of each muscle group weight is unexpectedly small. After 8 month it becomes a little large, but relatively the younger the fetal age the larger the difference, as shown in Table 4 and 5; for example, in 4 month the individual difference reaches about 4 times between individuals on upper limb but in 10 month it is one and half times at the most. This is just contrary to the result mentioned above (2), namely, speed of growth being the rapidest from 9 to 10 month. It is presumed that the difference of few days of fetal growth is greatly concerned, because, the younger the fetal age, the faster the growth relatively.
(6) Tendency of index according to fetal age gradually increases on arm extensor index, gradually decreases on forearm extensor index, rapidly increases on thigh extensor index after 8 month, gradually decreases on adductor index, slightly increase both on leg extensor and fibular index, and these tendencies seem to continue in general to adult stage. However, in adult, thigh extensor index increases more remarkably than fetus and adductor index seems to increase.
(7) In comparison of monkey with man (including adult and fetus), monkey's arm extensor index is bigger than man, while forearm extensor index is smaller.
The most striking contrast between man and monkey is shown in the extensor and adductor index of the thigh; in man, these two muscle groups are heavier than the flexor, but in monkey the latter is heavier than the former two. This is the most remarkable difference between man, walking bipedal, and monkey, a quadrupedal animal in principle. Monkey's leg extensor index is bigger than man and fibular index stands amid between fetus and adult.
Thigh flexor and extensor are not studied in this report ; this is one of our next subjects together with the study of other quadrupedal animals.