The Body: A Guide for Occupants

The cerebrum “fills most of the cranial vault and is the part that we normally think of when we think of ‘the brain” (51). It is divided into left and right hemispheres, each side further divided into lobes. Behind the forehead are the frontal lobes, which deal with thought and personality; above and behind them are the parietal lobes, which handle bodily sensations; below the parietals are the temporal lobes, which deal with sound and language; in the back of the cerebrum are the occipital lobes, which take care of visual inputs.

Chromosomes—so named for their ability to absorb chemical dyes—are the strands of DNA in the nuclei of cells that determine the structure and makeup of the body. “A length of DNA is divided into segments called chromosomes and shorter individual units called genes” (6). Human eggs and sperm carry 23 chromosomes, which pair up to make a total of 46; one pair contains either two X chromosomes or one X and one Y, making the organism either female or male, respectively. Genes are sequences of DNA within chromosomes that determine specific traits, like eye color or blood type, within the body.

Along with phrenology, craniometry was a popular 19th-century practice that made “precise and comprehensive measurements of volume, shape, and structure of the head and brain” in an attempt to determine underlying characteristics of the head’s owner (73), such as personality, tendencies toward vice and crime, level of intelligence, etc. Though craniometrists regarded phrenologists with contempt, essentially their pursuits were aimed at similar ends and were equally unscientific. They are not to be confused with the modern science of craniology, used by anthropologists and paleontologists to categorize ancient peoples, and by forensic scientists to determine sex, age, and other characteristics of skulls recovered at, for example, crime scenes. 

Cytokines are “attack chemicals” fired during a body’s battle against infection. Sometimes, the immune system overreacts to a threat, as with allergies and certain varieties of infection, becoming “so ramped up that it brings out all its defenses and fires all its missiles in what is known as a cytokine storm” (203). The result can be deadly: The 1918 flu pandemic may have killed many young adults from cytokine storms. More recent diseases like avian flu, Ebola, and others can trigger the often fatal response. 

The “formal name for the skin” (11), the cutaneous system covers the body to protect it, provide cushioning, and respond quickly to invading diseases. The top layer, or epidermis, is basically dead skin, which tends to flake off over time, contributing to indoor dust. Beneath the epidermis is the dermis, or live skin, which also includes blood vessels, sebaceous glands that produce skin oils, sweat glands, and follicles that grow hair. Beneath the dermis is a layer of fat that cushions the body and “attaches the skin to the body beneath” (12).

A gland is “is any organ in the body that secretes chemicals” (150). The best known are the endocrine glands, which include the pituitary, pineal, hypothalamus, thyroid, parathyroid, thymus, pancreas, adrenals, and testes and ovaries. Each of these emits hormones, the chemicals that signal the body to perform particular actions. The pancreas, for example, emits insulin and other chemicals that regulate blood sugar levels; the testes send out testosterone, which affects male physique and personality. The liver, the largest gland and heaviest organ in the body, has multiple duties, including filtering the blood, storing food energy, and producing bile for digestion. 

Originally published in London in 1858 as Anatomy: Descriptive and Surgical by Dr. Henry Gray, with 363 illustrations by medical student Henry Vandyke Carter, the first definitive book on anatomy has been known ever since as Gray’s Anatomy and is used to this day by doctors in England and America. The 41st edition was published in 2015. Henry Gray tried successfully to minimize illustrator Carter’s contribution, but Gray didn’t live long enough to enjoy the fame, dying in 1861 of smallpox. Carter went on to a distinguished medical career, but it is Gray who is remembered. 

Immunotherapy involves “using the body’s own immune defenses to fight diseases” (210). It trains the immune system either to act on diseases it has overlooked or to stop overreacting to innocent antigens. Cancers, for example, can trick the immune system into ignoring them, but checkpoint therapy overrides this trick, permitting the immune response to resume attacking the cancer. CAR T-cell therapy alters T-cells genetically so they can kill cancers they had previously disregarded. These treatments also can cause serious side effects. 

Macronutrients are things the body needs to ingest in visible amounts: “water, carbohydrates, fat, and protein” (230). These are distinguished from micronutrients. 

Micronutrients are tiny amounts of organic chemicals and minerals that the body requires but can’t produce on its own and must ingest from food. Micronutrients include vitamins, a hodge-podge of organics, and some non-vitamin molecules such as choline. 

Melanin, one of the chemicals that gives skin its color, protects against the damaging rays of the sun; the more melanin in the skin, the darker the skin gets. Humans living in areas of more intense sunlight tend to have darker skin. Though they are simple anatomical adaptations to help with survival in various environments, the skin tones evoked by melanin have become rallying points for different groups of people who use skin color to rank and control others.

Microbes are mainly one-celled organisms, visible only under a microscope, that exist in profusion on Earth. They include bacteria, archaea, protozoa, fungi, and viruses. A small fraction of microbes cause most of the communicable diseases that afflict humans. About half of all the cells in a person’s body are microbial; the gut houses most of these, where they help with digestion. Though tiny, microbes together make up over 95% of all terrestrial life. 

Pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage” (306). It comes in four types: nociceptive, from a stubbed toe, cut, or other injury; inflammatory, from tissue that’s swollen and red; dysfunctional, or pain for no reason; and neuropathic, from injured nerves. Acute pain comes from injuries that later heal, while chronic pain tends to come from nerves that switch on and don’t switch off. Pain in internal organs generally isn’t felt in the organ itself but is shifted to another body part. This “referred” pain can show up, for example, in the arm or neck pain of a heart attack.

White blood cells “are vital for fighting off infections” (127). They are at the center of the immune system’s response to microbial or chemical invaders. The five types are “lymphocytes, monocytes, basophils, neutrophils, and eosinophils” (201). Lymphocytes, created in bone marrow, come in two types, B and T. T-cells mature in the thymus, an organ in the chest, then emerge to fight pathogens. Helper T-cells muster other immune cells; killer T-cells strike directly at invaders; and memory T-cells recall previous illnesses and act quickly to stop repeat invaders, which is why people become immune to some diseases. T-cells call on B-type lymphocytes to produce antibodies that attack invading microbes.