IRRATIONAL QUANTITIES IRRITABILITY. 



Their numbers have much diminished. Some of 

 the tribes are extinct ; some have made considerable 

 advances in civilization, while others have fallen into 

 a state of squalid misery. Some of the nations re- 

 mained in New York; others removed to Canada. 

 The number in New York, in 1818, was 4575, includ- 

 ing the Moheakunnuk or New Stockbridge, the 

 Mohicans and Narragansetts, who had been adopted 

 into die confederacy. They owned 265,315 acres of 

 land. See Colden's History of the Five Nations ; 

 Morse's Report on Indian Affairs, New Haven, 1822; 

 Indians, and Indian Languages, 



IRRATIONAL QUANTITIES are those which 

 cannot be measured by unity or parts of unity ; for 

 example, the square root of 2, 1-4124 .... which, 

 by continued approximation, can be obtained more 

 and more exactly, without end, in parts of unity, but 

 can never be exactly determined. The relation of 

 two quantities is also called irrational, when one can- 

 not be exactly measured by the whole and parts of 

 the other. The circumference and the diameter of a 

 circle stand in such an irrational relation to each 

 other, because we can only find by approximation, 

 how many times the latter is contained in the former. 



IRRAWADDY, or IRAWADDY ; a large river 

 of Asia, in the Chinese and Birman empires. Craw- 

 furd (Embassy to Ava, London, 1829) thinks it has 

 its source in the provinces of Lao and Yunan. Ac- 

 cording to Wilcox, it is eighty yards broad in lat. 

 27 30 , where he visited it, and he was informed 

 by the natives that he was fifty miles from its source. 

 It falls, by fourteen mouths, into the bay of Bengal, 

 after having divided into two principal branches, in 

 Pegu, lat. 17 9 45'. The most easterly branch passes 

 by Rangoon; the most westerly, by Bassien or Per- 

 saim. According to Crawfurd, it is navigable for 

 boats to Bhamo, about 300 miles above Ava. The 

 intermediate space between the eastern and western 

 branches forms a Delta, covered with trees and long 

 grass, and inhabited chiefly by buffaloes, deer, and 

 tigers. In lat. 21 45', it receives the Keen-Dwem, 

 a considerable river, from the north-west. 



IRRITABILITY (irritabilitas ; from irrito, to 

 provoke; vis insita of Haller; vis vitalis of Gorier; 

 oscillation of Boerhaave; tonic power of Stahl; mus- 

 cutar power of Bell; inherent power of Cullen); the 

 contractility of muscular fibres, or a property peculiar 

 to muscles, by which they contract, upon the applica- 

 tion of certain stimuli, without a consciousness of 

 action. This power may be seen in the tremulous 

 contraction of muscles when lacerated, or when en- 

 tirely separated from the body in operations. Even 

 when the body is dead, to all appearance, and the 

 nervous power is gone, this contractile power 

 remains till the organization yields, and begins to be 

 dissolved. It is by this inherent power that a cut 

 muscle contracts, and leaves a gap, that a cut artery 

 shrinks, and grows stiff after death. This irritability 

 of muscles is so far independent of nerves, and so 

 little connected with feeling, which is the province 

 of the nerves, that, upon stimulating any muscle by 

 touching it with caustic, or irritating it with a sharp 

 point, or driving the electric spark through it, or 

 exciting with the metallic conductors, as those of 

 silver or zinc, the muscle instantly contracts, although 

 the nerve of that muscle be tied ; although the nerve 

 be cut so as to separate the muscle entirely from all 

 connexion with the system; although the muscle be 

 separated from the body; although the^ creature, 

 upon which the experiment is performed,"may have 

 lost all sense of feeling and have been long apparently 

 dead. Thus a muscle, cut from the limb, trembles 

 and palpitates a long time after ; the heart separated 

 from the body, contracts when irritated; the bowels, 

 when torn from the body, continue their peristaltic 



motion so as to roll upon the table, ceasing to answer 

 to stimuli only when they become stiff and cold. 

 Even in vegetables, as in the sensitive plant, this con- 

 tractile power lives. Thence comes the distinction 

 between the irritability of muscles and the feasi- 

 bility of nerves; for the irritability of muscles survives 

 the animals, as when it is active after death; survives 

 the life of the part or the feelings of the whole sys- 

 tem, as in universal palsy, where the vital motions 

 continue entire and perfect, and where the muscles, 

 though not obedient to the will, are subject to 

 irregular and violent actions; and it survives the 

 connexion with the rest of the system, as when 

 animals very tenacious of life, are cut into parts; but 

 sensibility, the property of the nerves, gives the 

 various modifications of sense, as vision, hearing, and 

 the rest; gives also the general sense of pleasure or 

 pain, and makes the system, according to its various 

 conditions, feel vigorous and healthy, or weary and 

 low. The eye feels and the skin feels ; but their 

 appointed stimuli produce no motion in these parts : 

 they are sensible, but not irritable. The heart, the 

 intestines, the urinary bladder, and all the muscles of 

 voluntary motion, answer to stimuli with a quick and 

 forcible contraction; and yet they hardly feel the 

 stimuli by which these contractions are produced, or, 

 at least, they do not convey that feeling to the brain. 

 There is no consciousness of present stimulus in those 

 parts which are caljed into action by the impulse of 

 the nerves, and at the command of the will; so that 

 muscular parts have all the irritability of the system, 

 with but little feeling, and that little owing to the 

 nerves which enter into their substance; while nerves 

 have all the sensibility of the system, but no motion. 

 After every action in an irritable part, a state of rest, 

 or cessation from motion, must take place before the 

 irritable part can be again incited to action. If, by 

 an act of volition, we throw any of our muscles into 

 action, that action can only be continued for a certain 

 space of time. The muscle becomes relaxed, not- 

 withstanding all our endeavours to the contrary, and 

 remains a certain time in that relaxed state, before it 

 can be again thrown into action. Each irritable part 

 has stimuli which are peculiar to it, and which are 

 intended to support its natural action : thus blood is 

 the stimulus proper to the heart and arteries ; but if, 

 by any accident, it gets into the stomach, it produces 

 sickness or vomiting. The urine does not irritate 

 the tender fabric of the kidneys, ureters, or bladder, 

 except in such a degree as to preserve their healthy 

 action; but if it be effused into the cellular membrane, 

 it brings on such a violent action of the vessels of 

 these parts, as to produce gangrene. Such stimuli 

 are called habitual stimuli of parts. Each irritable 

 part differs from the rest in regard to the quantity of 

 irritability which it possesses. This law explains to 

 us the reason of the great diversity which we observe 

 in the action of various irritable parts: thus the 

 muscles of voluntary motion can remain a long time 

 in a state, of action, and, if it be continued as long as 

 possible, another considerable portion of time is 

 required before they regain the irritability they lost; 

 but the heart and arteries have a more short and 

 sudden action, and their state of rest is equally so. 

 The circular muscles of the intestines have also a 

 quick action and short rest. The action of every 

 stimulus is in an inverse ratio to the frequency of its 

 application. A small quantity of spirits, taken into 

 the stomach, increases the action of its muscular coat, 

 and also of its various vessels, so that digestion is 

 thereby facilitated. If the same quantity, however, 

 be taken frequently, it loses its effect. In order to 

 produce the same effect as at first, a larger quantity 

 is necessary; and hence the origin of dram-drinking. 

 The more the irritability of a part is accumulated, 



