A person beginning to study anatomy is struck first of all by the copi­ous terminology that must be firmly understood and completely mastered by every student and physician. These terms usually designate spatial rela­tions, the shape or size of various organs, and so forth. In mathematics and physical geography, certain initial points and planes are accepted from which distances are measured to establish spatial relations. In anatomy as well, there are generally accepted designations of perpendicular planes by means of which the position of organs or their parts in space is determined exactly. Three such planes are of primary impor­tance: sagittal, frontal, and horizontal. It should be borne in mind that the planes are related to an erect human body (Fig. 10). The sagittal plane is a vertical plane by means of which we divide the body mentally (for example, a fixed, frozen cadaver) with an arrow (L sa-gitta arrow) piercing it from front to back and with an arrow along the length of the body. The sagittal plane that passes through the middle of the body and divides it into two symmetrical, right and left, parts is called the me­dian plane (L medius middle) (Fig. 11). A plane drawn also vertically but at right angles to the sagittal plane is called the frontal plane and is paral­lel to the forehead (L frons forehead). The frontal plane divides the body into the front and back parts. The third, horizontal, plane in accordance with its name passes horizontally, i.e. at right angles to both the sagittal and the frontal planes. It divides the body into the upper and lower halves. The positions of the different points or lines in these planes are desig­nated as follows: those located nearer to the median plane are called medial (medialis) (L medius middle); those located further from the median plane are designated lateral (lateralis) (L latus side). Points and lines found on a front to back plane are designated as follows: those located nearer to the front surface of the body are called anterior or ventral (ventralis) (L venter belly); those nearer to the back are known as posterior or dorsal (dorsalis). The following points and lines are distinguished in the vertical plane: those nearer to the upper end of the body are called upper, superior, or cranial (cranialis) (Gk kranion skull); those nearer to the lower end are referred to as lower, inferior or caudal (caudalis) (L cauda tail). The terms proximal and distal are used in reference to the parts of the limbs. Proximal (L proximus nearest) is applied to parts nearer to the point of origin of the limb from the trunk; distal, in contrast, is a term applied to parts farthest from the trunk (L distare to be distant). On the upper limb, for instance, the elbow is proximal in comparison to the fingers, while the latter are distal in relation to the elbow. The terms external (externus) and internal (internus) are used mainly to designate positions in relation to the body cavities and whole organs, either outward or inward; the terms superficial (superficialis) and deep (pro­fundus) are used for organs located "less deep" or "more deep" in relation to the surface of the body or organ. The commonly used terms for size are great (magnus); small (parvus); greater (major); lesser (minor). The last two terms, major and minor, are used to designate the comparative size of two related or identical structures, e.g. tuberculum majus and minus on the humerus. The term magnus (great) does not imply the presence of another identical but smaller structure. For instance, nervus auricularis magnus, the great auricular nerve, is called so because of its thick trunk; there is no nervus auricularis parvus. The shapes of the various structures, particularly in osteology, have many designations whose meaning is best mastered during study of these structures.