An Introduction to physiology

of the muscular response, i.e. it indicated the character of the downward impulses, and may therefore be appealed to as an indicator of the passage of the upward impulses, which can otherwise only be studied by their reflex effects. More recently still, Beck has made observations similar to those of Caton; and v. Fleischl, in consequence of the results said to have been obtained by Beck, has published observations relating to the cerebral currents on man. There is no doubt that the current of action can be demonstrated in the white matter of the cord, as in a nerve-trunk; its manifestation by the grey matter of nervecentres is still sub judice.

Retinal currents. If two unpolarisable electrodes connected with a galvanometer are brought in contact with an excised frog's eye, one with the cornea, the other with the optic nerve, a deflection is obtained showing that the optic nerve is negative to

the cornea. This is an ordinary current of injury arising from the cut optic nerve, and not assignable to the retina. If an isolated eyeball, or better, an isolated retina, is put in connection with the galvanometer, and kept in the dark, a deflection will be obtained indicating that a current traverses the retina from without inwards, i.e. that the rod layer is negative to the fibre layer. If now the retina is illuminated for 15 to 20 secs., this current will be increased (positive variation) at commencement and at end of illumination; during illumination it is less increased, or in many cases actually diminished (negative variation).

If, while the current of injury is led off from the optic nerve, the retina be illuminated for a short period of time by opening a shutter, the current will be found to undergo a variation-(1) at the commencement, (2) during the incidence, (3) at the termination of the illumination, as represented in fig 191.

An entirely satisfactory explanation of these effects cannot be given, but the important fact remains that light acting upon the retina causes an electrical change which is the physical token of

retinal activity. Holmgren showed by exclusion that the electrical variation caused by light depends upon the retina only, and is not due to the action of light upon any other constituent of the eyeball; nor is it due to changes of pigment. He showed also that the external or choroidal surface of the retina is negative to its internal surface. Kühne and Steiner found that the variation occurs nearly as well on a retina devoid of visual purple as on a non-bleached retina, although its character was somewhat different in the two cases. Dewar and McKendrick found that

white or coloured light acted upon the retina in the following order of strength-white, yellow, green, red, blue. They also demonstrated the electrical variation on intact animals and on man, one electrode being on the eye, the other on any other part of the body. With regard to the magnitude of the change, they estimated it at 10 Daniell. The essential phenomenon is the current of action; whether or no a current pre-exists, due to injury or other accidental conditions, is a matter of secondary import

ance.

The most striking manifestations of animal electricity are those that are afforded by electric fishes-gymnotus, torpedo, and malapterurus. Their electric organs are practically powerful batteries, from which a succession of instantaneous discharges of high intensity can be liberated at will of the animal, or in a reflex manner by cutaneous stimulation, or experimentally by direct electrical excitation of the organ itself or of its efferent nerves. An electrical organ is in effect a terminal apparatus of nerve, analogous with muscle, giving action currents of very great intensity, and, like muscle, alkaline in the normal state, acid after death or excessive activity. In the torpedo it is formed of two

lateral masses composed of numerous hexagonal prisms perpendicular to the surface of the body, these prisms being subdivided into compartments by transverse septa parallel with the surface. In the discharge of the organ the current flows from the dorsal

FIG. 192.-DIAGRAMMATIC TRANSVERSE SECTION OF TORPEDO.

D dorsal, V ventral surface; Direction of discharge indicated by arrows.
(Du Bois-Reymond.)

to the ventral surface through the galvanometer, from the ventral to the dorsal surface through the organ itself. According to du Bois-Reymond, the electrical resistance of the living organ is very much greater to down currents from dorsal to ventral sur

FIG. 193.-GYMNOTUS.

Direction of discharge indicated by arrows and leading-off electrodes from water. face than to up currents from ventral to dorsal surface. According to Gotch, this apparent difference is the effect of an upward action current adding to the up current and subtracting from the down current.

In the gymnotus the shocks are always directed from tail to head in the animal, in the malapterurus they are directed from head to tail. The direction, as was pointed out by Pacini, depends upon the points of entrance of the electric nerves; in the torpedo they are distributed to the ventral surface, in the gymnotus to the posterior surface, and in the malapterurus to the anterior surface of the organ. The discharge is always such that the nerve-surfaces are active,.

FIG. 194.-MALAPTERURUS.

i.e. analogous with the zine of a Daniell cell. The discharge is a discontinuous one, composed of as many as 200 shocks per second in the fresh condition, but with exhaustion falling in

Led off by two shields; discharge indicated by frequency and in force. A arrows. (Du Bois-Reymond.) single shock has a latent

period of about second, and a duration of second. The rate of transmission along the electrical nerves is about 7 meters. per second; the fish themselves are unaffected by electrical currents sufficient to kill non-electrical fish.

400 Reflection-Refraction-Index of refraction-The prism.

401

Lenses-Foci-Principal focus-Conjugate focus-Real focus-Virtual focus
Centre of curvature-Optical centre-Principal axis-Secondary axis.

402 Images-Real reversed and virtual erect-Mirror images.

403 Spherical aberration-Chromatic aberration.

404 Lens units-The Diopter.

COLOUR

404 Composition and decomposition of white light.

405 Complementary colours-Differences of colour-Tone, saturation, and brightness—* The colour triangle-Young-Helmholtz theory-Hering's theory. 412 Colour-blindness-The colour-perimeter.

THE EYE

Iris-Pupil-Retina-Refractive media-The lens.

416 Listing's reduced eye: Nodal point-Optic axis-Line of vision-Visual angle-Optical problems.

419 Accommodation: Ciliary muscle-Far point-Near point-Range of accommodation-Variations with age-Myopia, hypermetropia, and presbyopia -Sanson's images.

424 The field of vision-The perimeter.

424

Scheiner's experiment.

427 Astigmatism-Irradiation.

428 The Ophthalmoscope: Indirect method-Direct method.

431 Relation of retina to field of vision --Hemiopia and hemianopsia.