(_b_) Ataxia is a symptom of the utmost importance, and its strict definition should be maintained. It is that form of inco-ordination in which there is want of harmony in the action of the various muscular groups employed in the voluntary performance of a given act. The simplest movement of an extremity (and more complicated ones a fortiori) is made up of simultaneous contractions of flexor and extensor muscles, often also of adductors and abductors in due proportion, so as to produce a steady position and movement of the parts. This is not accomplished by the will acting directly on the different muscles themselves, but through the medium of a co-ordinating mechanism which is strictly spinal (including the intracranial expansion of the cord as far frontad as the third ventricle), and which is educated or trained from the first days of life. We will the action and set the co-ordinating mechanism going, so that the movement is in a certain sense indirectly accomplished. Yet volition does interfere directly to a certain extent by estimating through the muscular sense, and by exerting the proper amount of force required, and by guiding the movement in a general way. The most striking peculiarity about ataxic movements is the defect in the harmonious action of the various {48} (antagonistic) muscles employed, resulting in jerky, oscillatory, but non-rhythmic movements, constantly made worse by greater tension of the will. In simple tremor volition may for a few moments stop the movements (as best shown in paralysis agitans), while in ataxia the more the patient tries to achieve the act, the greater becomes the disorder. The anatomical fault is mostly in the spinal co-ordinating mechanism; the long-established motor and sensori-motor associations are interrupted or confused, and in some cases besides the muscular sense is impaired.[5] The idea that ataxia results from faulty centripetal impressions due to various degrees of anæsthesia is certainly erroneous and misleading. In the first place, there are cases recorded in which typical ataxia of the extremities occurred without the slightest impairment of sensibility; and, secondly, neither in animals (section of posterior roots of spinal nerves) nor in man does ataxia appear as a result of anæsthesia. In this condition there is inco-ordination, to be sure, but not ataxia in the strict meaning of the term. Ataxia is nearly always a symptom of organic disease of the spinal cord, more especially posterior spinal sclerosis and diphtheritic myelitis. It may also occur from disease of the oblongata, pons, and crura cerebri. In certain cases of lesion in caudo-lateral part of the thalamus involving the caudal part of the internal capsule (irritation or partial destruction), we observe various forms of irregular movements which have been designated by Weir Mitchell as post-paralytic chorea. We long ago became satisfied that this term included quite a number of forms of inco-ordination,