(_a_) Diminution and abolition of reflex action are frequent symptoms of disease of the nervous system. The fault or break may be anywhere in the reflex arc, so that each case must be analyzed by itself. Let us consider the phenomena as exhibited by two widely distant and different apparatuses. First, in the eye. In case of atrophy of the optic nerve the pupillary reflex is lost, the reflex action failing because the receptive surface and efferent nerve are injured. In certain cases of spinal disease (posterior spinal sclerosis) the same pupillary immobility is observed (the {52} Argyll-Robertson pupil); and in this case the lesion either affects indirect efferent spinal fibres destined for the iris, or it is situated in the centre for the reflex action—viz. the gray matter of the lobus opticus. There may be loss of pupillary reflex due to injury of the direct efferent fibres of the arc (paralysis of the motor oculi, N. iii.). Lastly, the iris itself may be so diseased as to be incapable of contracting, though it receive the reflex impulse properly; the lesion is then in the terminal organ of the arc. Second, the patellar tendon reflex. In a healthy individual, sitting at ease with one leg thrown over the other (knee over knee), upon tapping the ligamentum patellæ of the overhanging or free leg a contraction of the quadriceps muscle occurs, causing a visible forward movement of the leg and foot. This is the well-known patellar reflex or knee-jerk. The arc in this case consists of the ligamentum patellæ with its included sensory fibres as receptive organs, sensory (afferent) fibres of the crural nerve, a segment of the lumbar gray matter of the cord as centre, motor (efferent) fibres of the crural nerve, supplying the quadriceps extensor femoris, which is the terminal organ. Theoretically (vide Fig. 1), we can conceive of numerous abnormal conditions of parts of this arc which would lead to abolition of the patellar reflex, but in practice the following are the principal lesions to be thought of: Disease (sclerosis) of the posterior root-zones of the lumbar enlargement of the cord, as exhibited in the pre-ataxic stage of tabes; disease of the posterior roots themselves through meningitis or meningo-myelitis, as in diphtheritic ataxia; lesion of the nervous centre, as is frequently observed in cases of infantile poliomyelitis; a lesion of the crural nerve, involving its efferent or afferent fibres, or both sets of fibres, would produce the same result, as would also, lastly, a severe myositis or cancerous infiltration of the quadriceps muscle.