_The first centres to be attacked are those of thought and perception._ The patient is incapable of coherent reasoning, and loses touch to some extent with impressions from the outside world. _Muscular sense and co-ordination next become affected._ Although still able to move the limbs or the head, movements are incoherent, and if at this stage the patient were put upon his feet, he would stagger as he does in alcoholic intoxication. By this time _sensation_, both tactile and special, begins to be affected. The patient is no longer cognisant of pain,--if cut he would at any rate not have a remembrance of pain. _The special senses_ are at this stage also lost, one of the last to go being the auditory sense, a point which is sometimes forgotten by those inclined to talk while anæsthesia is being induced. _Muscle tone is the next function_ to be lost, and at this stage all movements on the part of the patient should cease except those of respiration. _The reflexes_ disappear at varying stages: the spinal reflexes, _e.g._ the knee-jerks, disappear fairly early, probably before muscle tone is entirely abolished, but certain other reflexes persist to a later stage. Those which are of most interest to the anæsthetist are the conjunctival, corneal, and pupillary reflexes of which he will find full details in Chapter V. Lastly the _vital medullary centres_, respiratory, vasomotor, and cardiac are overcome, and at this stage we have passed beyond the stage of a proper anæsthesia into that of over-dosage. In passing it may be observed that the level at which one endeavours to work is that indicated by the loss of muscle tone and of some of the reflexes and the full activity of the medullary centres, and that an anæsthetic is good or bad according as it gives a wide or narrow margin between these two events. Upon the _peripheral nerves_, anæsthetics have much less effect than on the central nervous system. Faradisation of a _motor nerve_ will in the deepest anæsthesia still cause immediate contraction of the muscles supplied by it, showing that the conductivity of the nerve is unaffected. Of far more importance, however, is the fact that the _sensory nerves_ are not paralysed. That pain is not felt by the patient is due simply to the loss of function of the cerebral sensory centres; _injury to the nerve still causes an impulse to be transmitted to the brain_. Since no operative procedure can be carried out without more or less trauma (injury) to sensory nerves, we may picture the brain of the patient undergoing a surgical operation while under a general anæsthetic, as being constantly bombarded by sensory stimuli, which though not consciously appreciated by the sleeping patient, are yet capable of producing reflex effects of a definite character, the importance of which to the work of the surgeon and anæsthetist it is difficult to exaggerate, and of which a condensed account will be found in the succeeding chapter.