The Science of Yoga

and can rotate on its axis about 50 degrees. Yoga practitioners typically move the vertebrae much farther. For instance, an intermediate student can easily turn his or her neck 90 degrees—nearly twice the normal rotation.
    Russell had long specialized in understanding how the bending of the neck could endanger the flow of blood from the heart to the brain. His concern focused mainly on the vertebral arteries. By nature, every tug, pull, and twist of the head rearranges these highly elastic vessels. But major activity outside their normal range of motion can put them in jeopardy in part because of their unusual structure.
    In traversing the neck, the vertebral arteries go through a bony labyrinth that is quite unlike anything else in the body and quite different from the soft, easy path that the carotids follow to the brain. The sides of each vertebra bulge outward to form loops of bone, and the arteries penetrate these loops successively in moving upward. The left and right vertebral arteries enter this gauntlet at C6 and run straight through the loops until they reach the top of the neck, at which point they start to zig and zag back and forth as they move toward the skull. Between C2 and C1, they usually bendforward, and then, upon exiting the bony rings of C1, usually curve sharply backward toward the foramen magnum—the large hole at the base of the skull that acts as a conduit for not only blood vessels but nerves, ligaments, and the spinal cord. Anatomists describe the final journey of the vertebral arteries toward the brain as serpentine and report much variability in the exact route from person to person. It is not unusual for the tops of the vertebral arteries to branch out in a tangle of coils, kinks, and loops.
    From decades of clinical practice and laboratory study, Russell knew that extreme motions of the head and neck could wound these remarkable arteries, producing clots, swelling, constriction, and havoc downstream in the brain. The victims could be quite young. His ultimate worry centered on the basilar artery. Located just inside the foramen magnum, the vessel arises from the union of the two vertebral arteries and forms a wide conduit at the base of the brain that feeds such structures as the pons (which plays a role in respiration), the cerebellum (which coordinates the muscles), the occipital lobe of the outer brain (which turns eye impulses into images), and the thalamus (which relays sensory messages to the outer brain and the hypothalamus and its vigilance area). In short, the basilar artery nourishes some of the brain’s most important areas. Russell worried that clots and cutoffs of blood in the vertebral arteries would impair the work of the basilar artery and its downstream branches deep inside the brain.
    The drop in blood flow was known to produce a variety of strokes. Symptoms might include coma, eye problems, vomiting, breathing trouble, arm and leg weakness, and sudden falls—but by definition had little to do with language and conscious thinking. However, because strokes of the rear brain can severely damage the regulatory machinery that governs life basics, they can also result in collapse and death. Even so, the vast majority of patients survive the attack and go on to recover most functions. Unfortunately, in some cases, headaches can persist for years, along with such residual troubles as imbalance, dizziness, and difficulty in making fine movements.
    The medical world of Russell’s day worried about these kinds of strokes, including a prominent type that began in circumstances that seemed quite innocuous. At beauty salons, during shampooing, women at times would have their necks tipped too far back over the edge of a sink, reducing the flowof blood through the vertebral and basilar arteries. The risk was judged especially great among the elderly. With aging, the vertebral arteries can lose their elasticity and narrow, and the normally smooth neck bones can grow spurs. When the neck bends far backward, the bony spurs can compress or otherwise harm vessels already narrow and inelastic. In addition, the stagnant blood can turn into a small factory of clot production. When the neck returns to a more normal position and the flow of blood resumes, the clots can travel down the arteries, heading deeper into the brain before settling in a narrow vessel and blocking its flow. A small epidemic of strokes resulted in a diagnosis known as the beauty-parlor syndrome.
    Russell warned of