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GENERAL PHYSIOLOGY. 



and respiration. In these various states and acts, the animal exercises 

 the functions of sensation, the psychical functions, and those of the 

 regulation of the muscular movements, all of which are accomplished 

 through the agency of the nervous apparatus, consisting of the brain 

 and spinal cord, of certain ganglia, and of the numerous connected 

 cords formed by the nerves. In the exercise of the various kinds of 

 sensation, the animal feels, tastes, smells, hears, and sees, by means 

 of the vibrissse, tongue, nose, ears, and eyes, organs of special sense, 

 furnished with appendages for their protection and more efficient use, 

 and destined to receive impressions made by various external stimuli, 

 the effects of which are transmitted by special nerves to the great 

 organ of sensation, the brain. In this organ, also, not only conscious- 

 ness of the sensations, but all other psychical phenomena, have their 

 corporeal seat ; such as perception of the outward causes of the sen- 

 sations, ideas, emotions, desires, reasoning processes, and will, the 

 stimuli or mandates of all which latter proceed from the brain to the 

 muscles, destined to perform the necessary ideational, emotional, or 

 voluntary acts. In other cases, stimuli produce impressions on the 

 nerves, which are conveyed to the spinal cord, or to its extension up- 

 wards into the head, named the "medulla oblongata," and do not 

 induce sensation, but are reflected outwards involuntarily along other 

 nerves to particular muscles, which then contract and perform the 

 necessary movements. There exist accordingly, in the animal organ- 

 ism, sensory nerves and sensorial nervous centres, motor nerves and 

 motorial nervous centres; and there are also found nervous centres 

 concerned in the reflected or reflex motor actions of the body. 



These mixed motor, sensory, and psychical functions, which consti- 

 tute the so-called proper animal functions, cannot be performed con- 

 tinuously without cessation. The animal sooner or later becomes 

 exhausted in regard to them ; the brain becomes weary and the mus- 

 cles fatigued. Rest is indispensable. These functions are for a time 

 suspended, and the condition known as sleep occurs, which, when per- 

 fect, is accompanied by the temporary suspension of all these animal 

 functions motor, sensory, and psychical. But sleep is insufficient of 

 itself permanently to restore animal activity. In the exercise of the 

 muscular and nervous systems, these organs undergo a destruction or 

 waste of their component molecules. During every interval of rest, 

 as well as during sleep, they are renovated by materials derived from 

 the common nutrient circulating fluid, the blood. But the blood it- 

 self, in thus contributing to restore the wasted nervous and muscular 

 organs, itself becomes impoverished. Under these circumstances, it 

 may, for a time, draw material from one part of the body to sustain, 

 as long as it can, another part, the more passive organs of the frame 

 yielding nutriment to those more actively endowed. But the waste 

 still goes on in proportion as action is renewed ; fatigue again ensues ; 

 rest and sleep are once more indispensable. The animal day by day 

 would emaciate, get weaker, and ultimately die of inanition. To pre- 

 vent this, new material must be added to the blood from the outer 

 world. This material consists of food, drink, and air ; and to impel 

 the animal to seek them, the special feelings of hunger and thirst arise 



