An annual curve may be found in the expulsive force of the bladder as measured by the distance to which the urinary stream can be projected. This curve, as ascertained for one case, is interesting on account of the close relationship between sexual and vesical activity. After a minimum point in autumn there is a rise through the early part of the year to a height maintained through spring and summer, and reaching its maximum in August.[171] This may be said to correspond with the general tendency found in some cases of nocturnal seminal emissions from a winter minimum to an autumn maximum. There is an annual curve in voluntary muscle strength. Thus in Antwerp, where the scientific study of children is systematically carried out by a Pedological Bureau, Schuyten found that, measured by the dynamometer, both at the ages of 8 and 9, both boys and girls showed a gradual increase of strength from October to January, a fall from January to March and a rise to June or July. March was the weakest month, June and July the strongest.[172] Schuyten also found an annual curve for mental ability, as tested by power of attention, which for much of the year corresponded to the curve of muscular strength, being high during the cold winter months. Lobsien, at Kiel, seeking to test Schuyten's results and adopting a different method so as to gauge memory as well as attention, came to conclusions which confirmed those of Schuyten. He found a very marked increase of ability in December and January, with a fall in April; April and May were the minimum months, while July and October also stood low.[173] The inquiries of Schuyten and Lobsien thus seem to indicate that the voluntary aptitudes of muscular and mental force in children reach their maximum at a time of the year when most of the more or less involuntary activities we have been considering show a minimum of energy. If this conclusion should be confirmed by more extended investigations, it would scarcely be matter for surprise and would involve no true contradiction. It would, indeed, be natural to suppose that the voluntary and regulated activities of the nervous system should work most efficiently at those periods when they are least exposed to organic and emotional disturbance.