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Show 1 9 0 3 . ] TRANSPOSITION OF MAMMALIAN TESTES. 3 2 3 limbs make contact with the earth, when, as already described, the entire portion of the trunk is suddenly upheaved with considerable force. If the horizontal velocity could be reckoned as constant, the only accelerations which the trunk would acquire would be in a vertical line; but since the horizontal velocity is a quantity which, owing to the resistance of the air, is continually decreasing, the upward acceleration suddenly imparted to the trunk at each contact of the limbs with the earth has to be compounded with a sudden forward acceleration in order to maintain the pace, and hence the resultant acceleration which the half of the body acquires at every upheaval is in an upward and forward direction. It is most essential to recognise this fact that the body as a whole has periodically imparted to it large accelerations in these two directions. As is obvious, the upward and horizontal velocities due to these accelerations become respectively neutralised by gravity and atmospheric resistance at the middle and end of the interval which exists between successive contacts of the limbs with the earth. From the instant when the maximum elevation of the half of the trunk is attained, i. e. when the upward velocity is completely neutralised, until contact again occurs, the body continues to acquire a downward acceleration due to gravity. But on contact taking place, the large downward velocity due to this acceleration is suddenly destroyed, and as suddenly exchanged for an equal upward velocity, in the brief instant of time occupied by the impact of limbs and earth. Whence it is evident that the degree of upward acceleration imparted to the half of the body during the sudden loss and gain of downward and upward velocities must be considerable. It is noteworthy that whereas each forward acceleration, due to either pair of limbs, is imparted to the body as a wdiole, each itpioard acceleration is solely imparted to that portion of the trunk to which the pair of limbs causing the elevation belongs; and hence the attachments of those organs situated in the median portion of the trunk are evidently not so subject to downward strains as in the case of organs placed more terminally. Thus it is important to notice, with regard to what follows, that the primitive pre-mammalian portion of the testes is decidedly posterior (see diagram, p. 334), and that in consequence the motion of the hind portion of the body is the factor of most importance in the consideration of the present problem. From the foregoing, it is sufficiently manifest that at each elevation of the body in mammals a considerable force is imparted to the organisation as a whole (and by necessary implication to its constituent parts) in an upward and forward direction-as was to be proved. If the constituent parts of the body be considered, it will be evident that such a force works differential effects among them. For these organs, largely differing among themselves in regard to mass, and the same force being incident on all, it follows that the individual reactions are different, and hence there exists a tendency to segregation of those organs of greater mass from those of less. Moreover, organs differ inter se in their relation |
