Stereognosis and Stereoisomerism

STEREOGNOSIS (from the Greek stereos—solid and gnosis—knowledge), stereognostic sense, is the ability to recognize objects by touch. It is based on those sensations which are derived from the form, size, warmth, density, and character of the surface of the object being felt. Stereognosis is built upon complex perception, a combination of tactile and temperature sensations with deep muscle sense, the sensation of movements of the feeling parts (fingers), and the connection of the images arising during palpation with visual and kinesthetic memory images. Thus, stereognosis is a complex function, depending not only on the activity of peripheral and central sensory systems but also connected with the synthetic activity of the cerebral cortex. In the brain, the lower part of the middle third of the central gyri, the supramarginal gyrus, the angular gyrus, and the occipital lobe are related to stereognosis. In right-handed people, the left hemisphere is of greater importance for stereognosis than the right. Stereognosis is examined in such a way that a patient with closed eyes is given small objects one by one into their hand (a coin, a key, a pencil, geometric figures) and is asked, while feeling them, to describe them as accurately as possible. A disorder of stereognosis is called astereognosis (see). Wernicke first pointed out in 1895 that the disorder of stereognosis does not run parallel to the disorder of elementary sensitivity and has a specific localization in the cerebral cortex. Monakow and others reject the localization of stereognosis, considering it too complex a process. The school of Weizsäcker, which uses more refined methods for examining sensitivity than those accepted until now in clinical practice, calls into question "pure stereognosis," i.e., the possibility of a disorder of stereognosis without a disorder of elementary sensitivity; STEREOISOMERISM, STEREOISOMERS. Stereoisomerism is isomerism caused by the different arrangement of atoms or atomic groups in space. A distinction is made between: 1) Optical isomerism or mirror isomerism—caused by the presence of an asymmetric carbon atom in the molecule, which leads to the formation of stereoisomers—two spatial isomers, where one is, as it were, a mirror image of the second (see figure under Asymmetric carbon). With an increase in the number of asymmetric carbon atoms in a molecule, the number of possible stereoisomers also grows—the number of the latter is equal to 2ⁿ, where n is equal to the number of asymmetric atoms. 2) Geometric isomerism, caused by the different arrangement of groups in relation to the plane formed either by a double bond or a cycle (see Stereochemistry).