Anatomy

• 1. Introduction

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  Anatomy traces its beginnings to prehistoric times, making it one of the oldest of the natural sciences. The word itself comes from the Greek anatome, meaning dissection, from the verb anatemno, "I cut up, cut open." For most of its history, the only way to learn how a body was built was to open one. Condemned criminals supplied the corpses. Grave robbers raided cemeteries at night. Watchtowers rose over burial grounds to guard the dead. How did a discipline born from cutting open carcasses and cadavers become a field where machines now see inside the living without a single incision? Who first dared to dissect a human, and why did that act remain taboo for over a thousand years? And what does it mean that the limb of a fish, a frog, and a human all share the same underlying skeleton? These are the questions this documentary follows, from the body of a paramecium to the structure of the human hand.

• 2. Cutting And Seeing

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  Anatomy is the branch of morphology concerned with the internal and external structure of organisms and their parts. It examines the appearance and position of those parts, the materials they are made from, and how they relate to one another. It stands apart from physiology and biochemistry, which deal with how those parts function and the chemical processes inside them. An anatomist studying the liver asks about its shape, size, position, blood supply, and innervation. A physiologist studying the same liver asks how it produces bile and regulates the body.

  Gross anatomy, also called macroscopic anatomy, is the study of structures large enough to be seen with the naked eye. It includes superficial or surface anatomy, the study by sight of external body features. Microscopic anatomy goes further, using optical instruments to examine tissues and cells. Within it sit histology, the study of tissues, and embryology, the study of an organism in its immature condition. Regional anatomy looks at all the structures in one body region, such as the abdomen. Systemic anatomy follows a single body system, such as the digestive system, wherever it runs.

  Dissection means opening a body and studying its organs. Endoscopy threads a video-camera instrument through a small incision to explore internal organs. Angiography uses X-rays or magnetic resonance to make blood vessels visible. The word anatomy usually calls to mind the human body, but similar structures run throughout the animal kingdom. The term zootomy refers specifically to non-human animals, while the dissimilar structures of plants belong to plant anatomy. That separation of plant from animal sets up a deeper question about what all animal bodies share.

• 3. The Four Tissues

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  Animals are multicellular organisms that are heterotrophic and motile, though some have secondarily adopted a sessile life. Most have bodies differentiated into separate tissues and are known as eumetazoans. Their cells lack the cell wall and chloroplasts of plant cells, and any vacuoles are smaller and more numerous. Every cell typically carries a phospholipid membrane, cytoplasm, and a nucleus. All of an animal's cells trace back to the embryonic germ layers. Diploblastic animals form from two layers, ectoderm and endoderm. Triploblastic animals form from three, adding the mesoderm.

  Connective tissue is fibrous, built from cells scattered through an extracellular matrix. It is often called fascia, from the Latin word for band or bandage, and it gives organs their shape and holds them in place. Its most abundant protein is collagen, which organizes and maintains tissues. The matrix can harden into a skeleton, whether the rigid external cuticle of a crustacean or insect, or the internal endoskeleton of more developed animals.

  Epithelial tissue is composed of closely packed cells bound together with little space between them, resting on a basal lamina. These cells can be squamous, cuboidal, or columnar, and they take on many forms. The respiratory tract carries a ciliated lining, the small intestine has microvilli, and the large intestine has villi. Skin is keratinized stratified squamous epithelium, and keratinocytes make up to 95 percent of its cells.

  Muscle cells, or myocytes, form the contractile tissue that produces force and motion. Smooth muscle has no striations and contracts slowly, found in sea anemone tentacles and the body wall of sea cucumbers. Skeletal muscle contracts rapidly over a limited range, moving appendages and jaws. Cardiac muscle exists only in the heart, pumping blood through the body. The obliquely striated muscle of earthworms sits between the fast and slow types.

  Nervous tissue is made of neurons that transmit information. In slow-moving, radially symmetrical animals such as ctenophores and cnidarians, the nerves form a loose nerve net. In most animals they bundle longitudinally. Higher animals develop a central nervous system of brain and spinal cord, plus a peripheral nervous system. That peripheral system splits into the somatic, which controls voluntary muscle, and the autonomic, which involuntarily governs smooth muscle and internal organs. These four tissue types are the raw material from which every vertebrate body is assembled.

• 4. The Shared Body Plan

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  Every vertebrate shares the major chordate characteristics at some point in life, usually in the embryo. There is a stiffening rod called the notochord, a dorsal hollow neural tube, pharyngeal arches, and a tail behind the anus. The spinal cord runs above the notochord, protected by the vertebral column, while the gut runs below. Nervous tissue comes from the ectoderm, connective tissue from the mesoderm, and the gut from the endoderm.

  The vertebral column itself, a segmented series of vertebrae, is what defines a vertebrate. In most species the notochord becomes the nucleus pulposus of the intervertebral discs, but a few, including the sturgeon and the coelacanth, keep the notochord into adulthood. Jawed vertebrates carry paired appendages, fins or legs, which some lineages have secondarily lost. The limbs of vertebrates are considered homologous, built on the same skeletal structure inherited from a last common ancestor. Charles Darwin put this forward as one of the arguments supporting his theory of evolution. That single inherited blueprint is what makes the bodies of fish, amphibians, reptiles, birds, and mammals worth comparing side by side.

• 5. Variations On A Spine

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  A fish body divides into head, trunk, and tail, though the boundaries are not always visible from outside. The skeleton is cartilage in cartilaginous fish or bone in bony fish, with the vertebral column as its main element. The heart has two chambers, pumping blood through the gills and around the body in a single loop. A lateral line of sense organs runs along each side, detecting low-frequency vibrations and changes in water pressure. Sharks and rays keep primitive features, including cartilage skeletons, placoid scales, and a cloaca, but no swim bladder. Bony fish carry a swim bladder to hold depth and broadcast many small eggs into the water.

  Amphibians comprise frogs, salamanders, and caecilians, with skin that holds little keratin and lacks scales but carries mucous and sometimes poison glands. Their hearts have three chambers, two atria and one ventricle. They breathe by buccal pumping, drawing air into the buccopharyngeal region and forcing it into the lungs, and they supplement this with gas exchange through moist skin. Frogs have powerful hind legs and no tail. Caecilians are limbless and resemble earthworms, burrowing by waves of muscle contraction.

  Reptiles comprise turtles, tuataras, lizards, snakes, and crocodiles, with horny scales that waterproof the skin and eggs wrapped in amniotic membranes laid on land. Most have a heart partly divided by a septum, but crocodilians have four chambers that fully separate oxygenated and deoxygenated blood. Turtles carry a carapace above and a plastron below, partially fused with the ribs and spine. The tuatara has only one living species, Sphenodon punctatus, with lineages that diverged from lizards in the Triassic, and it carries a well-developed parietal eye on its forehead. Snakes branched from lizards in the Cretaceous, lost movable eyelids, and gained jaws flexible enough to swallow prey whole.

  Birds are tetrapods whose front limbs became feathered wings. Their long bones are thin, hollow, and light, with air-sac extensions from the lungs filling some of them. They have no teeth, only a horn-covered beak, and their single skin gland is the uropygial gland near the tail, which oils and waterproofs the feathers during preening.

  Mammals carry hair, sweat glands, and mammary glands that produce milk. They have three middle-ear bones, a cochlea, a four-chambered heart, and a muscular diaphragm separating thorax from abdomen. Most are viviparous, but the egg-laying monotremes, the platypus and the echidnas of Australia, are exceptions. In marsupials the young is born immature and completes development in the mother's pouch, latched to a teat. Humans share this overall mammalian plan, with a head, neck, trunk, two arms, and two legs, and that shared plan is what medical students spend their first year learning to take apart.

• 6. Life Without A Backbone

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  Invertebrates make up about 95 percent of all animal species, ranging from single-celled eukaryotes such as Paramecium to the octopus, lobster, and dragonfly. By definition, none has a backbone. In single-celled protozoans, parts of the cell specialize into the equivalent of tissues and organs. Locomotion comes from cilia, flagella, or advancing pseudopodia, and some protozoans form multicellular colonies.

  Epithelium and connective tissue, the two most basic metazoan tissues, are present in nearly all invertebrates. Skeletons take many forms across these groups. An endoskeleton from the mesoderm appears in echinoderms, sponges, and some cephalopods. Exoskeletons of chitin armor the arthropods, calcium carbonate builds the shells of molluscs and brachiopods, and silica forms the exoskeletons of microscopic diatoms and radiolaria. Softer invertebrates secrete coatings instead, such as the gelatinous cuticle of cnidarians or the collagenous cuticle of annelids.

  Arthropods form the largest phylum of invertebrates, with over a million known species. Insects carry a chitin exoskeleton and a body in three parts, head, thorax, and abdomen. The thorax bears three pairs of legs and one or two pairs of wings, and the abdomen is composed of eleven segments housing the digestive, respiratory, excretory, and reproductive systems. Spiders, a class of arachnids, have four pairs of legs, two body segments, no wings, and no antennae. Their chelicerae often connect to venom glands, and their pedipalps serve as organs of taste and smell.

  Marcello Malpighi, the father of microscopical anatomy, found that plants had tubules like those he saw in insects such as the silk worm. When a ring of bark was removed from a trunk, he watched a swelling form in the tissue above the ring. He read this as growth fed by food coming down from the leaves and captured above the cut. That observation, made through a lens, points toward the longer struggle to win the right to look inside the body at all.

• 7. From Grave Robbery To Imaging

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  In 1600 BCE, the Egyptian Edwin Smith Papyrus described the heart and its vessels, the brain and its meninges, the cerebrospinal fluid, and organs including the liver, spleen, kidneys, uterus, and bladder. The Ebers Papyrus, from around 1550 BCE, carried a treatise on the heart and the vessels feeding every member of the body. Centuries later, Hellenistic Alexandria became the stepping-stone for Greek anatomy, home to the largest medical library in the world and richly patronized by the Ptolemaic dynasty.

  Herophilus and Erasistratus, two physicians of the third century, pioneered human dissection using the cadavers of condemned criminals, an act considered taboo until the Renaissance. Herophilus was recognized as the first person to perform systematic dissections. He classified the pulse, found that arteries had thicker walls than veins, named the meninges and ventricles of the brain, and recognized the brain as the seat of intellect, rejecting Aristotle's idea that it was a cooling chamber. Erasistratus described the structure of the brain, distinguished cerebrum from cerebellum, separated sensory from motor nerves, and named the epiglottis and the heart's valves, including the tricuspid.

  In the second century, Galen of Pergamum wrote the final and most influential anatomy treatise of ancient times, drawn mostly from dog dissection. His drawings served as effectively the only anatomical textbook for the next thousand years, surviving in the Islamic Golden Age until translation from Greek in the 15th century. As the historian Marie Boas wrote, progress before the sixteenth century was as mysteriously slow as its development after 1500 was startlingly rapid. Between 1275 and 1326, Mondino de Luzzi and colleagues at Bologna carried out the first systematic human dissections since ancient times, and Mondino's Anatomy of 1316 became the standard textbook for a century.

  Leonardo da Vinci, who lived from 1452 to 1519 and was trained in anatomy by Andrea del Verrocchio, dissected humans and other vertebrates and sketched their skeletons, muscles, and organs. Andreas Vesalius, who lived from 1514 to 1564 and held the chair of anatomy at the University of Padua, is considered the founder of modern human anatomy. His De humani corporis fabrica, published in seven volumes in 1543, carried intricate illustrations thought to be the work of Jan van Calcar, a pupil of Titian.

  Demand for cadavers turned grim. Philadelphia, Baltimore, and New York became known for body snatching, and in Britain grave-raiding and even anatomy murder were practiced to obtain corpses. The Anatomy Act of 1832 halted the trade in Britain. In the United States, similar legislation followed after William S. Forbes of Jefferson Medical College was found guilty in 1882 of complicity with resurrectionists in despoiling graves at Lebanon Cemetery. Sir John Struthers, Regius Professor of Anatomy at Aberdeen from 1863 to 1889, built the system of three pre-clinical years of science teaching and became famous for his public dissection of the Tay Whale. Ignaz Semmelweis traced puerperal fever to students moving from the dissecting room to the maternity ward, and showed that washing hands in chlorinated lime sharply reduced the deaths.

  The microscope changed what counted as anatomy. Around 1839, Matthias Jakob Schleiden and Theodor Schwann identified cells as the fundamental unit of all living things, and the microtome was invented to cut tissue thin enough to examine. The electron microscope later opened the ultrastructure of cells, and in the 1950s X-ray diffraction of proteins and nucleic acids gave rise to molecular anatomy. Today X-rays, magnetic resonance imaging, computed tomography, and ultrasound reveal the body's interior to a degree far beyond the imagination of the grave robbers who once supplied the dissecting table.