The Science of Yoga

robbing the brain of oxygen. By definition, strokes traumatize and kill brain cells, which are known as neurons. A renewed flow of blood can sometimes mend beleaguered cells. And over time, nearby neurons can sometimes replace the function of dead cells. But damage can also be permanent. Stroke victims thus experience disabilities that range from passing weakness to lasting neurologic damage to death if the destruction involves vitalbrain centers. (Fast treatment can limit the damage, which is why health professionals urge speedy evaluations of suspected stroke victims, preferably within sixty minutes.) The symptoms of stroke vary widely because of the brain’s highly specialized anatomy. For instance, conscious thought and intelligence arise in the outer layers of the brain, so strokes in those areas can affect speech and critical thinking.
    Russell’s concern went deeper. He worried about the inner brain, in particular a functionally diverse region toward the rear. His concern was that yoga postures that involved extreme bending of the neck might compromise the region’s blood supply, destroying parts of the brain rich in primal responsibilities.
    The human neck is made of seven cervical vertebrae that anatomists have numbered, top to bottom, C1 through C7. Their special shapes and compliant disks make the neck the most flexible part of the spinal column. Scientists have measured the neck’s normal range of motion and found the movements to be extraordinarily wide. The neck can stretch backward 75 degrees, forward 40 degrees, and sideways 45 degrees, 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