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perfect, as for instance the brain and spinal medulla, it requires only a closer study to reveal many points of difference between the right and left halves.
The line on the front of the body along which the median plane reaches the surface is termed the anterior median line; whilst the corresponding line behind is called the posterior median line.
It is convenient to employ other terms to indicate other imaginary planes of section through the body. The term, sagittal, therefore, is used to denote any plane which cuts through the body along a path which is parallel to the median plane (SS); and the term coronal or frontal is given to any vertical plane which passes through the body in a path which cuts the median plane at right angles (C C'). The term horizontal, as applied to a plane of section, requires no explanation.
Any structure which lies nearer to the median plane than another is said to be medial to it; and any structure placed further from the median plane than another is said to lie lateral to it. Thus in Fig. 1, A is lateral to B; whilst B is medial to A.
The terms internal and external are applied to the walls of hollow cavities or organs; thus, the ribs possess external surfaces, that is, surfaces away from the cavity of the thorax, and internal surfaces adjacent to the cavity.
The terms anterior and ventral are synonymous, and are used to indicate a structure (D) which lies nearer to the front or ventral surface of the body than another structure (E) which is placed nearer to the back or dorsal surface of the body, and which is thus said to be posterior or dorsal. In some respects it would be well to discard the terms "anterior" and "posterior" in favour of "ventral" and "dorsal," seeing that the former are only applicable to man in the erect attitude, and cannot be applied to an animal in the prone or quadrupedal position. They are, however, so deeply ingrained into the descriptive language of the human anatomist that they cannot be entirely discarded. A similar objection may be raised to the terms superior and inferior, which are employed to indicate the relative levels at which two structures lie with reference to the upper and lower ends of the body. The equivalent terms of cephalic or cranial and preaxial are, therefore, sometimes used in place of "superior," and caudal and postaxial in place of "inferior."
The terms proximal and distal should be applied only in the description of the limbs. They denote relative nearness to or distance from the root of the limb. Thus, the hand is distal to the forearm, whilst the arm or brachium is proximal to the forearm.