352 SPECIAL PHYSIOLOGY. 



of attention given by the mind to such internal sensations, would oc- 

 cupy it, so as to interfere with its perception of external phenomena. 

 In weak conditions of health, in sudden excitement, and in certain 

 diseases, however, the sensation of palpitation of the heart is not un- 

 common. The sensation of fluttering at the pit of the stomach is 

 usually not cardiac, but either diaphragmatic or gastric. Heartburn 

 is also erroneously named ; for it is a gastric sensation, dependent on 

 the accumulation of acid, or other acrid fluid in the stomach. In mor- 

 bid conditions, actual pain is felt in the heart; and, in mental states, 

 a sense of weariness is referred to that region. Morbid sensations of 

 heat or chilliness along the veins, or in the blood, as such feelings are 

 termed, are probably owing to conditions, not of the vessels or blood, 

 but of the nerves of common sensation, especially of those of the skin. 



Of the internal sensations connected with the respiratory organs, by 

 far the most powerful is that known under the name of the feeling of 

 want of breath. It is this which creates an irresistible desire to inhale 

 air, and an uncontrollable involuntary effort to inspire. Its final 

 cause is undoubtedly to maintain life, which is so immediately depen- 

 dent upon the continuance of the respiratory function. It is difficult 

 to determine the precise seat to which this sensation is referred ; but 

 it is chiefly located in the larynx and neighboring parts of the throat, 

 and also in the anterior part of the chest and precordia, opposite the 

 level of the diaphragm ; it is probably, in great part, a muscular sen- 

 sation. Its cause is, however, general, and perhaps depends on the 

 presence of an undue quantity of carbonic acid in the blood, which, 

 reaching the nervous centres concerned in receiving impressions from 

 the respiratory organs, and in governing the respiratory movements, 

 at once excites the sensation of distress, and the involuntary impulse 

 to inspire, which is best calculated to relieve it. The sensation of 

 want of breath is named apncea. Difficulty of breathing is named 

 dyspnoea, an embarrassing sensation also referred to the larynx, throat, 

 and chest. These conditions may be produced by diminution in the 

 number and depth of the respirations, by breathing a vitiated atmo- 

 sphere, by obstacles to the introduction of fresh air into the lungs, as 

 in asthma and oedema of the larynx, by diminution of the respiratory 

 surface, as the result of disease, and by defective action of the blood 

 as a conveyer of oxygen. 



With regard to the muscular sensations of the contractile organs 

 employed in the functions of digestion, circulation, and respiration, 

 it is remarkable that the respiratory muscles, although they are iden- 

 tical in structure with the rest of the voluntary muscles, and are them- 

 selves subject to the will, and, though they are constantly in action 

 during the whole of life, yet do not habitually, and in healthy con- 

 ditions, convey the sense of fatigue to the mind. As regards the 

 heart, no direct sense of fatigue in its muscular walls is experienced. 

 So, also, the movements of the stomach and intestines are usually un- 

 perceived. In inflammation of any of the parts just considered, as of 

 the intercostal muscles, the diaphragm, the heart, and the alimentary 

 canal, acute pain, spasm, or cramp, is not uncommon. 



The organs of vegetative life are, it would seem, in health, possessed 



