| It is, however, of much advantage to add to the facts ascertained by naked-eye dissection those obtained by the use of the microscope. This introduces two fields of investigation, viz., the study of the minute structure of the various component parts of the bodyhistologyand the study of the human organism in its immature condition, i. e., the various stages of its intrauterine development from the fertilized ovum up to the period when it assumes an independent existenceembryology. Owing to the difficulty of obtaining material illustrating all the stages of this early development, gaps must be filled up by observations on the development of lower formscomparative embryology, or by a consideration of adult forms in the line of human ancestrycomparative anatomy. The direct application of the facts of human anatomy to the various pathological conditions which may occur constitutes the subject of applied anatomy. Finally, the appreciation of structures on or immediately underlying the surface of the body is frequently made the subject of special studysurface anatomy.
| SYSTEMATIC ANATOMY.The various systems of which the human body is composed are grouped under the following headings:|
|Osteologythe bony system or skeleton.|
|Syndesmologythe articulations or joints.|
|Myologythe muscles. With the description of the muscles it is convenient to include that of the fasciæ which are so intimately connected with them.|
|Angiologythe vascular system, comprising the heart, bloodvessels, lymphatic vessels, and lymph glands.|
|Neurologythe nervous system. The organs of sense may be included in this system.|
|Splanchnologythe visceral system. Topographically the viscera form two groups, viz., the thoracic viscera and the abdomino-pelvic viscera. The heart, a thoracic viscus, is best considered with the vascular system. The rest of the viscera may be grouped according to their functions: (a) the respiratory apparatus; (b) the digestive apparatus; and (c) the urogenital apparatus. Strictly speaking, the third subgroup should include only such components of the urogenital apparatus as are included within the abdomino-pelvic cavity, but it is convenient to study under this heading certain parts which lie in relation to the surface of the body, e. g., the testes and the external organs of generation.
| For descriptive purposes the body is supposed to be in the erect posture, with the arms hanging by the sides and the palms of the hands directed forward. The median plane is a vertical antero-posterior plane, passing through the center of the trunk. This plane will pass approximately through the sagittal suture of the skull, and hence any plane parallel to it is termed a sagittal plane. A vertical plane at right angles to the median plane passes, roughly speaking, through the central part of the coronal suture or through a line parallel to it; such a plane is known as a frontal plane or sometimes as a coronal plane. A plane at right angles to both the median and frontal planes is termed a transverse plane.
| The terms anterior or ventral, and posterior or dorsal, are employed to indicate the relation of parts to the front or back of the body or limbs, and the terms superior or cephalic, and inferior or caudal, to indicate the relative levels of different structures; structures nearer to or farther from the median plane are referred to as medial or lateral respectively.
| The terms superficial and deep are strictly confined to descriptions of the relative depth from the surface of the various structures; external and internal are reserved almost entirely for describing the walls of cavities or of hollow viscera. In the case of the limbs the words proximal and distal refer to the relative distance from the attached end of the limb.