accommodation

a term used in biology and medicine, similar to “adaptation.” The term is traditionally used in the three phrases “accommodation of the eye,” “physiological accommodation,” and “histological accommodation.”

Accommodation of the eye is adaptation of the eye in order to obtain clear vision of objects at various distances. It is achieved by a change in the refractive power of the optical system, which leads to the focusing of the image on the retina. In fish the eye is constructed for close vision, and accommodation is achieved by the rearward displacement of the spherical crystalline lens. In amphibians and reptiles the eye at rest is constructed for distant vision, and accommodation is achieved by the forward movement of the crystalline lens. In birds, mammals, and humans accommodation is achieved by a change in the curvature of the crystalline lens. Accommodation is performed by the accommodation muscles and is possible within certain limits—that is, when the object is at a particular distance from the eye within the field between the points of close and distant vision.

When the intraocular, so-called accommodation muscles contract, the ligaments are relaxed and the crystalline lens suspended on them becomes more protuberant. An increase in the curvature of the crystalline lens leads to a stronger refraction, in consequence of which it becomes possible for the eye to focus the light rays emanating from closely placed objects on the retina and to see them clearly.

The physical factor of crystalline lens elasticity also enters into eye accommodation. In humans the ability of the accommodation muscles to contract and the elasticity of the crystalline lens may change because of a number of conditions. Accommodation is most highly developed in children. With age, elasticity of the crystalline lens diminishes, the ability to see close objects decreases, and so-called senile vision, or presbyopia, develops. Signs of diminution of eye accommodation in persons with normal refraction (emmetropia) appear most commonly between the ages of 40 and 45. When such diminution of eye accommodation occurs, glasses with convex lenses are prescribed for close work (reading and writing) to compensate for the defect. Subsequently, the strength of the eyeglass lenses is gradually increased; by the age of 60 or 70 eye accommodation is usually entirely lost and it is no longer necessary to increase the strength of the eyeglass lenses. Spasm of the accommodation muscles may occur as a result of prolonged eyestrain, bright light, eye trauma, or other factors. Paralysis of these muscles may occur with certain infections and intoxications—for example, syphilis, influenza, diabetes, diphtheria, and botulism. The treatment of spasm or paralysis depends on the cause.

Physiological accommodation, or accommodation of excited muscular or neural tissue, is adaptation to the action of a slowly intensifying stimulus. For example, rapid chilling or a blow may excite an isolated nerve fiber, whereas slow cooling or gradual pressure may not. The same thing is observed with electrical stimulation; with the slow intensification of current, the action potential of a nerve or muscle does not emerge. Such accommodation is associated with active changes in the tissue that raise the threshold of irritability and obstruct the development of excitation.

Histological accommodation, or tissue accommodation, is a change in the forms and relationships of tissue elements (cells) in the process of adaptation to changed conditions. An example of histological accommodation is the transformation of the cubical epithelium of the glomerular capsule into long cylindrical cells when the volume of the glomerulus decreases. However, histological accommodation is often difficult to differentiate from other processes, such as metaplasia and atrophy. The term is therefore merely a formal one.