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Updated: June 3, 2025
*Sensation Stimuli.*—While the sensations are dependent upon the afferent impulses, the afferent impulses are in turn dependent upon causes outside of the nervous system. If these are removed, the sensations cease and they do not start up again unless the exciting influences are again applied.
Of chief importance in the outer layer are the cells which are acted upon directly by the light and are named, from their shape, the rods and cones. In contact with these, but occupying a separate layer, are the ends of small afferent nerve cells. In the center of the retina is a slight oval depression having a faint yellowish color, and called, on that account, the yellow spot.
As a result of increased differentiation, the skin and the central nervous system became further and further separated, and in the end the two were only permanently connected by the afferent peripheral sensory nerves. The observations of the comparative anatomist are in complete accord with this view.
For example, with the instinct of pugnacity, we have the emotion of anger; with that of flight, the emotion of fear; with the parental instinct, the emotion of love or tender feeling. An instinct, therefore, is regarded as a mechanism made up of three parts: First, an afferent or cognitive part, through which it is stimulated.
After the establishment of these conditions, afferent impulses from the eyes, ears, skin, and other places, under the general direction of the cerebrum, may cause such actions as the balancing of the body, walking, etc., as well as the delicate and varied movements of the hand. This view of its functions makes of the cerebellum the great center of secondary reflex action.
Fischer in a personal letter says: "You have two possibilities for the production of glaucoma with sinus disease: A toxic factor due to poisons being carried into the eye; and second, interference with a proper blood supply to the eye through compression of the efferent or afferent blood vessels supplying the eye from edema of the tissues about the eye consequent upon the sinus infection.
The most common form of capillary angioma is the nævus or congenital telangiectasis. #Nævus.# A nævus is a collection of dilated capillaries, the afferent arterioles and the efferent venules of which often share in the dilatation. Little is known regarding the etiology of nævi beyond the fact that they are of congenital origin.
"The afferent nerves, when excited by some physical irritant, be this as gross in its mode of operation as a chopping axe or as subtle as the waves of light, convey the excitement to the nervous centres. The commotion set up in the centres does not stop there, but discharges through the efferent nerves, exciting movements which vary with the animal and with the irritant applied."
On the outside of the elastic membrane, just beneath the ectoderm, is a plexus or cobweb of nervous cells and fibrils. As in every nervous system, three elements are here to be found. 1. An afferent or sensory nerve-fibril, which under adequate stimulus is set in vibration by some cell of the epidermis or ectoderm, which is therefore called a sensory cell. 2.
The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord. The first convey impulses to the cord and are called afferent neurons; the second convey impulses from the cord and are known as efferent neurons. A division of this pathway reaches the brain.
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