Anatomy

H

The Reproductive System

Reproduction is accomplished by the union of male sperm and the female ovum. In coitus, the male organ ejaculates more than 250 million sperm into the vagina, from which some make their way to the uterus. Ovulation, the release of an egg into the uterus, occurs approximately every 28 days; during the same period the uterus is prepared for the implantation of a fertilized ovum by the action of estrogens. If a male cell fails to unite with a female cell, other hormones cause the uterine wall to slough off during menstruation. From puberty to menopause, the process of ovulation, and preparation, and menstruation is repeated monthly except for periods of pregnancy. The duration of pregnancy is about 280 days. After childbirth, prolactin, a hormone secreted by the pituitary, activates the production of milk.

I

Skin

The skin is an organ of double-layered tissue stretched over the surface of the body and protecting it from drying or losing fluid, from harmful external substances, and from extremes of temperature. The inner layer, called the dermis, contains sweat glands, blood vessels, nerve endings (sense receptors), and the bases of hair and nails. The outer layer, the epidermis, is only a few cells thick; it contains pigments, pores, and ducts, and its surface is made of dead cells that it sheds from the body. (Hair and nails are adaptations arising from the dead cells.) The sweat glands excrete waste and cool the body through evaporation of fluid droplets; the blood vessels of the dermis supplement temperature regulation by contracting to preserve body heat and expanding to dissipate it. Separate kinds of receptors convey pressure, temperature, and pain. Fat cells in the dermis insulate the body, and oil glands lubricate the epidermis.

III

History of Anatomy

The oldest known systematic study of anatomy is contained in an Egyptian papyrus dating from about 1600 bc. The treatise reveals knowledge of the larger viscera but little concept of their functions. About the same degree of knowledge is reflected in the writings of the Greek physician Hippocrates in the 5th century bc. In the 4th century bc Aristotle greatly increased anatomical knowledge of animals. The first real progress in the science of human anatomy was made in the following century by the Greek physicians Herophilus and Erasistratus, who dissected human cadavers and were the first to distinguish many functions, including those of the nervous and muscular systems. Little further progress was made by the ancient Romans or by the Arabs. The Renaissance first influenced the science of anatomy in the latter half of the 16th century.

Modern anatomy began with the publication in 1543 of the work of the Belgian anatomist Andreas Vesalius. Before the publication of this classical work anatomists had been so bound by tradition that the writings of authorities of more than 1000 years earlier, such as the Greek physician Galen, who had been restricted to the dissection of animals, were accepted in lieu of actual observation. Vesalius and other Renaissance anatomists, however, based their descriptions on their own observations of human corpses, thus setting the pattern for subsequent study in anatomy.

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A

Morphology

For many years anatomists (even those of the modern era) were concerned mainly with the accumulation of a vast amount of information known as descriptive morphology. Descriptive morphology has been supplemented, and to a certain extent supplanted, by the development of experimental morphology, which attempts to identify the hereditary and environmental determinants in morphology and their relationships by controlled-environment and grafting experiments on embryos. Ideally, anatomical investigation consists of a combination of descriptive and experimental approaches. Present-day anatomy involves scrutiny of the structure of organisms at many levels of observation. For example, the anatomist studies the cells and tissues of organisms with the unaided eye, with simple and compound lenses, with various kinds of microscopes, and by chemical methods of analysis.

B

Microscopic Anatomy

The 17th-century invention of the compound microscope led to the development of microscopic anatomy, which is divided into histology, the study of tissues, and cytology, the study of cells. Under the leadership of the Italian anatomist Marcello Malpighi, the study of the microscopic structure of animals and plants flourished during the 17th century. Many great anatomists of the period were reluctant to accept microscopic anatomy as part of their science. By contrast, modern anatomy is studied usually with the aim of correlating the structure of organisms as seen by the naked eye with their structure as revealed by more refined methods of observation.

Pathological anatomy was established as a branch of the science by the Italian physician Giovanni Morgagni, and in the late 18th century comparative anatomy was systematized by the French naturalist Georges Cuvier.

In the late 18th and early 19th centuries restrictive legislation limiting the use of unclaimed human bodies for the study of anatomy and surgery gave rise in England and the United States to an era of body snatching. The scandals arising from this practice forced the repeal of the English restrictions in 1832 and the enactment of more advanced legislation.

Microscopic anatomy developed rapidly in the 19th century. During the second half of the century many basic facts about the fine structure of organisms were discovered, largely as a result of greatly improved optical microscopes and of new methods that made cells and tissues easy to study with this instrument. The method of microtomy, the cutting of tissue into thin, practically transparent slices, was perfected. Microtomy was rendered incomparably more valuable by the application to the tissue slices of various types of dyes and stains that make it much easier to see various parts of the cell.

Knowledge of microscopic anatomy was greatly expanded during the 20th century as a result of the development of microscopes that provided much greater resolution and magnification than had conventional instruments, thus revealing formerly unclear or invisible detail; and expanded laboratory techniques helped facilitate observation. The ultraviolet microscope allows the observer to see more because the wavelengths of its probing rays are shorter than those of visible light (the resolving power of a microscope is inversely proportional to the wavelength of the light used). It also is used to emphasize particular details through selective absorption of certain ultraviolet wavelengths. The electron microscope gives even greater magnification and resolution. These tools have opened up formerly unexplored fields of anatomical investigation. Other modern microscopes have made visible unstained and living materials that would be invisible under the conventional microscope. Two examples are the phase-contrast microscope and the interference microscope. Through utilization of ordinary light beams, both these instruments differentiate parts of living, unstained cells.

The discovery of X rays by the German physicist Wilhelm Conrad Roentgen enabled anatomists to study tissues and organ systems in living animals. The first X-ray photograph, taken in 1896, was of a human hand. Today’s techniques permit three-dimensional X-ray photographs of the soft tissues of the viscera after ingestion of special opaque fluids, and of “slices” of the body with computer-aided X-ray beams. See Radiology. The latter is called computerized tomography, or CT scanning. Other noninvasive techniques that have been developed include the use of ultrasonic waves for imaging soft tissues and the application of nuclear magnetic resonance systems to research and diagnosis.

Another 20th-century technique of anatomical investigation is tissue culture, which involves the cultivation of cells and tissues of complex organisms outside the body. The technique permits the isolation of living units so that the investigator can directly observe the processes of growth, multiplication, and differentiation of cells. Tissue culture thus has added a new dimension to anatomical science.