The variation of intellectual function which appears in sleep serves to measure its profundity and to indicate the extent of its invasion. The act of perception being dependent upon sensation, it is to be observed that the range of perception diminishes so soon as the organs of sense begin to yield. Its intensity may not immediately fail, but the breadth of its scope is narrowed. Sometimes, however, the act of conscious perception is arrested before the organs of sense are sealed. The sleepy reader may continue to eye the page before him, perhaps even to read aloud for a considerable time after he has ceased to derive any meaning from the words of the book. In such cases the organs of perception and conception and association of ideas slumber before the bonds of connection between the will and the muscular organs have been completely relaxed. Such an example affords a valuable illustration of the division of the brain into separate mechanisms which, though most intimately related, are nevertheless partially independent of each other. Sleep may operate like an invasive disease, falling with unequal incidence upon the different structures that make up the mass of the brain, paralyzing one portion, while simply benumbing another, and even arousing to excessive activity a third. Consequently the intellectual functions may be very unequally disturbed, and the order of their subsidence may be considerably varied; but, as a general rule, the physiological relations of the faculties are respected, so that as sensation diminishes, perception fails, the conception of ideas is correspondingly hindered, and the association of such ideas as are still projected upon the field of consciousness becomes more imperfect. The loss of the power of association implies the destruction of memory and the impossibility of exercising the reasoning faculty or of forming those judgments upon which every act of volition is based. When the brain has at length been so far overwhelmed that physical impressions can no longer reach the field of consciousness, all manifestation of intellectual life is at an end, and the sleeper sleeps a dreamless sleep that leaves no trace behind.