Anatomy and Physiology

anatomy , 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...

brain the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function Occupying the skull cavity (cranium), the adult human brain normally weighs from 2 1/4 to 3 1/4 lb (1-1.5 kg). Differences in weight and size do not correlate with differences in mental ability; an elephant's brain weighs more than four times that of a human. In invertebrates a group of ganglia or even a single ganglion may serve as a rudimentary brain. By means of electrochemical impulses the brain directly controls conscious or voluntary behavior, such as walking and thinking. It also monitors, through feedback circuitry, most involuntary behavior—connections with the autonomic nervous system enable the brain to adjust heartbeat, blood pressure, fluid balance, posture, and other functions—and influences automatic activities of the internal organs. There are no pain receptors in brain tissue. A headache is felt because of sensory impulses coming chiefly from the meninges or scalp. Anatomically the brain has three major parts, the hindbrain (including the cerebellum and the brain stem ), the midbrain, and the forebrain (including the diencephalon and the cerebrum). Every brain area has an associated function, although many functions may involve a number of different areas. The cerebellum coordinates muscular movements and, along with the midbrain, monitors posture. The brain stem,...