The mechanism of ciliary movement.—VI. Photographic and stroboscopic analysis of ciliary movement

Abstract

When viewed under a suitable power of the microscope a normally moving cilium seldom, if ever, yields the impression of a well-defined image to the human eye. The blurred image, which is observed, is due to the idiosyncrasies of the eye rather than to the size or velocity of movement of the cilium. As explained elsewhere (Gray, 1928), the tip of an active cilium is seldom moving more rapidly than 3 inches a minute, but it is reversing the direction of its movement about 30 times per second. In order that the eye should register a clear image of a moving object, it is essential that the axis of the eye should move simultaneously in the direction of motion, for by so doing the image of the object is focussed on the same region of the retina for a requisite period of time. Since normal cilia reverse their direction of movement at a frequency far in excess of that which is possible for the axis of the human eye, a moving cilium registers, on the retina, a series of closely adjacent and super-imposed images which are responsible for the sensation of a blurred image. Hitherto the analysis of ciliary movement has been restricted to methods which are either concerned with the nett result of the activity of a ciliated epithelium or with the behaviour of individual cilia whose normal frequency has been artificially reduced to a very low level. By the use of a suitable cinematograph camera and a simple stroboscope this limitation of enquiry can be largely overcome. The first section of this paper deals with the development of the necessary technique and is followed by others which are concerned with specific problems of ciliary movement. The form and duration of the two phases of a single beat of an individual cilium can only be recorded photographically if the period of time involved is large in comparison to the duration of a single cycle of the camera being used. If the period of complete ciliary beat is 0·1 second, the minimum frequency of the cinematograph camera must be of the order of 100 exposures per second. So far, frequencies of this order have proved impracticable, for the apparatus available would not permit of frequencies higher than 24 per second. Fortunately, the normal duration of the beat of the large abfrontal cilia on the gills of Mytilus edulis is of the order of 0·5 second, and it has proved possible to take 12 photographs during this period. The apparatus used (Plate 21, fig. 1) is a modification of that supplied by Messrs. E. Leitz, and consists of an optical bench, microscope and camera mounted as one unit and suspended by vertical springs. As supplied by the makers, the apparatus does not record the frequency of the exposures and for this purpose two alternative methods have been employed. The first of these is illustrated diagrammatically in text-fig. 1. A small hole, L, is drilled in the base of an Ascania camera, and into it is fitted the lens of a 1-inch microscope objective to which is attached a metal sleeve; inside the sleeve is fitted a small 4-volt endoscope lamp (RL) whose dimensions are approximately 2 mm. by 1 mm. By suitable adjustment, the image of the lamp can be edge of the photographic film (F) as the latter passes over the continuously moving sprocket wheel, into the uptake drum (UD) of the camera. In the lamp circuit is placed the timing device, TD. The latter consists of an induction motor fitted with a brake and carrying in place of the usual steel two smaller concentric discs. One of these (SD) is painted white and is divided into 3° sectors by a series of black lines. The other disc (TD) is composed of two halves, one of brass and the other of hard rubber. The endoscope lamp circuit is made through the body of the motor at T and the sliding contact C. The whole apparatus is enclosed in a wooden box fitted with a window and illuminated from the inside by an alternating lamp (AL) of 60 cycles. In order to calibrate the time marker, the motor is switched on and the brake adjusted until the black lines on the disc (SD) appear stationary. At this point the inner disc TD is revolving once per second, and the endoscope lamp circuit is closed for half a second and open for half a second; with a reliable 60-cycle current, the timing of the motor was remarkably constant. On developing a strip of film, alternate bands of light and dark are found on the edge of the negative and the length between start of each dark band gives the length of film passing through the gate of the camera per second, and hence the interval between two successive photographs can readily be calculated. It is convenient to insert a relay (R) between the timing device and the lamp circuit, and to mount a pilot light (PL) in series with the endoscope lamp since the latter is not visible from the exterior of the camera. There is one objection to this type of time recorder: the small endoscope lamp takes a small but measurable time to glow on closing the current, so that lengths of film passing through the camera cannot be very accurately determined except over complete intervals of 1 second. This objection is eliminated in the second type of recorder.