=Preponderance of Cortex in Highest Animals.--=One of the most interesting conditions in the nervous system of the highest types of animals is the way in which the cortex has outrun the other parts of the brain in size and complexity and has come to dominate the organism more and more both directly and indirectly. Aside from the proportionately greater increase in size of the cortex, there is an abundance of anatomical evidence of this altered and probably altering system of control in man and the higher apes. This is well illustrated in the fiber tracts (nerve bundles) of the spinal cord. =More Long Fiber Tracts in the Spinal Cord of Man.--=The spinal cord although having many nerve centers of its own is also in great part a large cable for conducting enormous numbers of fibers from one part of the cord to another, or to and from the brain. In man and the higher apes a considerably larger percentage of the total area of the cord is given up to the long fiber tracts from the brain to the body than in lower vertebrates. This progressive increase in long fiber tracts in the higher anthropoids probably marks more and more domination of the body by the higher brain centers and correspondingly less by the direct activity of the cord and by the lower brain centers. However, even in man, many of the simpler reflexes of the body still have their centers in the spinal cord. =Special Fiber Tracts in the Cord of Man and Higher Apes.--=There are certain special tracts of the cord that are particularly interesting in connection with the increasing domination of the brain over the body, namely, the _pyramidal tracts_. These were the latest tracts to appear in the animal kingdom and are apparently the latest to become functional in the individual. It is believed that the development of the medullary substance (an enveloping sheath) of the common medullated nerve fiber marks the time of entrance of the fiber into activity and it is a significant fact that the formation of this sheath occurs last of all in the fibers of the pyramidal tracts, where it does not appear till after birth. These tracts convey impulses from the brain to the body. They consist of two sets of tracts, in fact, one the crossed, the other the direct. As an anomaly, probably arising most frequently from instrumental injury at birth, the pyramidal tracts fail to develop normally, with the distressing result that the infant, although possessing perfectly normal brain activity and normal spinal cord reflexes, is unable to exercise voluntary control of the body. In other words the condition, like hare-lip, is one of suppressed development. At least this seems to be the most plausible explanation of what is known as _Little's disease_. Such unfortunates usually die early although they may survive for a few years.