Cross references: On page 68, the book's "Principle 3" is that "The Brain's Circuits Are Crossed". This is confirmed for the sensory input by the picture on 385 and for the motor output on 367. What the discussion on 367 doesn't mention is that the motor nerves that don't cross in the pyramids connect with interneurons farther down in the spinal cord which do cross. But why do they cross?
One version of the generally accepted explanation for why the tracts cross is given by the link to the Washington Post article, below. If it doesn't work right away, try it again. Sometimes the server gets bogged down. Note: The graphic which was originally part of the link has been taken down. There are two problems with this version of the explanation, and my excuse for forwarding it is that it is readily available, provides a basis for discussion, and I couldn't find a better version online in a reasonable amount of time. The
first problem is that the motor tracts of fish are crossed, not uncrossed as is shown in the diagram. The second problem is that this explanation leans heavily on the fact that an optical lens reverses its image. The nervous systems of both the amphioxus and the acorn worm are also crossed even though the light receptors of the amphioxus are lensless and the acorn worm has no light receptors at all. The original version proposed by Cajal did not depend on the reversal of the visual image. In the original version, if an early animal, including an amphioxus or an acorn worm, were touched on the left side, the message would need to cross to the right side so that the muscles on the right side would contract to move the body away from the stimulus on the left side. The problem with this is that, in an aqueous environment, if the muscles on the right side contract, the animal will be propelled to the left, toward the stimulus. A long time ago I found a more convincing explanation. Unfortunately, I failed to photocopy the article, so the explanation below is from memory. It is widely accepted that the first nervous system evolved in a primitive coelenterate, among whose modern descendants are the sea anemone and the jelly fish, and that all more recently evolved systems evolved from this early coelenterate configuration. Although modern coelenterates have nervous systems that are more complex, this early nervous system is hypothesized to have been a network that looked something like this during the phase of the animal's life cycle when it remained pretty much stationary on the bottom.  There is a "node" at the base of each tentacle which sends two nerves diagonally down-across the body of the animal. There is an electrochemical synapse everywhere the nerves cross, and the nerves conduct equally well in both directions. The nodes provide both sensory input to, and motor output from, the nervous system. The nerve network allows the input at one node to influence the output from its neighbors. Since there are no nerves in the tentacles, the input to the nodes is chemical, primarily from the tentacle with which it is most closely associated. The input diffuses down the tentacle from stinging cells, which discharge on physical contact. The output from the node is also chemical, and as it diffuses back up the tentacle it informs the muscles when they need to contract because the stinging cells on the neighboring tentacles have been firing in large numbers. This is important information since it indicates that prey has been captured and is ready to be conveyed to the mouth. The coelenterates have radial symmetry and
consist of many identical units arranged side-by-side in a circle. The hypothesis suggests that the genetic guidance for this growth pattern has two components. One genetic component produces the basic structural element which is repeated, and the second genetic component controls the repetition. The hypothesis suggests that during the evolution of As you can see, the two nodes would become the two bilateral body form, the first genetic component was maintained and the second genetic component was modified so that there was but a single repetition. This results in two identical body structures, as produced by the first genetic component, lying side-by-side and interconnected more or less as would have been the case had the second genetic component allowed the repetition to continue. halves of the brain and each side of the brain would communicate primarily with the opposite side of the body. |