anatomy (әnăt´әmē), branch of biology concerned with the study of body structure of various organisms, including humans. Comparative anatomy is concerned with the structural differences of plant and animal forms. The study of similarities and differences in anatomical structures forms the basis for classification of both plants and animals. Embryology (see embryo) deals with developing plants or animals until hatching or birth (or germination, in plants); cell biology covers the internal anatomy of the cell, while histology is concerned with the study of aggregates of similarly specialized cells, called tissues. Related to anatomy is morphology, which involves comparative study of the corresponding organs in humans and animals. There are four major types of tissue present in the human body: epithelial tissue (see epithelium), muscular tissue (see muscle), connective tissue, and nervous tissue (see nervous system). Human anatomy is often studied by considering the individual systems that are composed of groups of tissues and organs; such systems include the skeletal system (see skeleton), muscular system, cutaneous system (see skin), circulatory system (including the lymphatic system), respiratory system (see respiration), digestive system, reproductive system, urinary system, and endocrine system. Little was known about human anatomy in ancient times because dissection, even of corpses, was widely forbidden. In the 2d cent., Galen, largely on the basis of animal dissection, made valuable contributions to the field. His work remained authoritative until the 14th and 15th cent., when a limited number of cadavers were made available to the medical schools. A better understanding of the science was soon reflected in the discoveries of Vesalius, William Harvey, and John Hunter. Various modern technologies have significantly refined the study of anatomy: X rays, CAT scans, and magnetic resonance imaging (MRI) are only several of the tools used today to obtain clear, accurate representations of the inner human anatomy. In 1994, for the first time, a detailed three-dimensional map of an entire human being (an executed prisoner who volunteered his body) was made available worldwide via the Internet using data from thousands of photographs, CAT scans, and MRIs of tiny cross sections of the body.