IRB 



IRR 



the weather opens, the ground ought to 

 be moderately moistened. The sun's 

 power should guide us to the frequency 

 and quantity ofwater; nor should its quali- 

 ty be overlooked : water from warm soils 

 will produce effects widely different from 

 the streams flowing out of clay lands, or 

 such as are impregnated with iron, &c. 

 the best water usually rises out of gravel- 

 ly or chalky lands It is better to throw 

 the water on early in the day, during cold 

 weather, in order that the grass may dry 

 well, and the danger apprehended from 

 frosty nights be obviated ; but in summer, 

 the watering should take place late in the 

 evening, whereby the ground will be cold, 

 without danger of scorching the plants. 



We have dwelt thus long on the sub- 

 ject of irrigation, under the conviction of 

 its extreme importance : the reader may, 

 under the head of AGRICULTURE, find 

 a few additional remarks, which were 

 given with the view to bringing all matters 

 relating to farming under one general 

 head, while we reserved this mechanical 

 part to be separately treated, under its 

 proper designation. 



IRRITABILITY, in physiology, is the 

 property peculiar to the muscles, by 

 which they contract upon the applica- 

 tion of certain stimuli, without a consci- 

 ousness of action. Haller and other phy- 

 siologists denominate that part of the hu- 

 man body irritable, which becomes short- 

 er by being touched : very irritable, if it 

 contracts upon a slight touch. They call 

 that a sensible part of the human body, 

 which, upon being touched, transmits the 

 impression of it to the mind : on the con- 

 trary, they call that insensible, which, be- 

 ing burnt, torn, cut, &c. occasions no sign 

 of pain or convulsion, nor any sort of 

 change in the situation of the body. It is 

 inferred that the epidermis is insensible ; 

 that the true skin is the most sensible part 

 of the body ; that the fat and cellular 

 membranes are insensible ; and the mus- 

 cular flesh sensible, the sensibility of 

 which he ascribes rather to the nerves 

 than the flesh itself. The tendons, hav- 

 ing no nerves distributed among them, 

 are deemed insensible. Irritability then 

 is the distinguishing characteristic be- 

 tween the muscular and cellular fi- 

 bres. Irritability differs from sensibility, 

 and is not proportioned to it: the intestines 

 are less sensible than the stomach, but 

 more irritable ; the heart is very irritable, 

 though it has but a small degree of sen- 

 sation. The laws of irritability, accord- 

 ing to Dr. Crichton, are : 1. 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 in- 

 cited 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 be- 

 comes relaxed, notwithstanding all our 

 endeavours to the contrary, and remains 

 a certain time in that relaxed state, before 

 it can be again thrown into action. 2. 

 Each irritable part has a certain portion 

 or quantity of the principle of irritability 

 which is natural to it, part of which it 

 loses during action, or from the applica- 

 tion of stimuli. 3. By a process, wholly 

 unknown to us, it regains this lost quantity 

 during its repose or state of rest. In or- 

 der to express the different quantities of 

 irritability in any part, we say that it is 

 either more or less redundant, or more 

 or less defective. It becomes redundant 

 in a part, when the stimuli which 

 are calculated to act on that part 

 are withdrawn, or withheld for a certain 

 length of time, because then no action 

 can take place ; while, on the other hand, 

 the application of stimuli causes it to be 

 exhausted, or to be deficient, not only by 

 exciting action, but by some secret in- 

 fluence, the nature of which has not yet 

 been detected ; for it is a circumstance 

 -extremely deserving of attention, that an 

 irritable part or body may be suddenly de- 

 prived of its irritability by powerful sti- 

 muli, and yet no apparent cause of mus- 

 cular or vascular action takes place at the 

 time. Thus a certain quantity of spirits 

 taken at once into the stomach kills al- 

 most as instantaneously as lightning does : 

 the same thing may be observed of some 

 poisons, as opium, laurel-water, the juice 

 of some poisonous vegetables, &c. 4. 

 Each irritable part has stimuli which are 

 peculiar to it ; and which are intended to 

 support its natural action : thus blood., 

 which is the stimulus proper to the heart 

 and arteries, if by any accident it gets 

 into the stomach, produces sickness or 

 vomiting. 5. 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 di- 

 versity which we observe in the action of 

 various irritable parts : thus the mus- 

 cles of voluntary motion can remain a long 

 time in a state of action, and if it be con- 

 tinued as long as possible, another consi- 

 derable 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 mus 

 cles of the intestines have also a quick ac- 



