ACTION AND USES OF MEDICINES. 149 



294, Depressants, 



Depressants proper are those agents which, such as bloodletting, 

 depress the nerve force and system generally. Under the general head 

 of Depressants, however, it is usual to include also Sedatives, which abate 

 the nervous force without causing such positive general depression, and 

 Nauseants, which, as the name expresses, lower the tone of the system by 

 producing nausea. 



It will readily be seen that, though a broad distinction may be drawn, 

 yet no sharply defined line divides these agents. 



295. Bleeding. 



Bleeding was some years ago considered to be amongst the most power- 

 ful and certain of the depressants, and was much employed by the older 

 practitioners. That it does act as a depressant is beyond doubt, but such 

 action mainly depends upon the weakening effects produced by the 

 sudden reduction of the quantity of blood in the circulation. The idea that 

 bleeding lessens the action of the heart is erroneous, because during the 

 act of bloodletting the heart beats are increased in frequency, but the 

 contractile force of its walls is weakened. Consequently the pulse, al- 

 though quickened, loses any vibratory hardness it may have, and becomes 

 softer in character. 



When it is thought necessary to employ it, blood enough should be 

 taken to produce a marked alteration in the character of the pulse. The 

 blood should be drawn in a full stream, so as to produce the effect as 

 quickly as possible. If bleeding is resorted to at all, it should be in the 

 very early stage of disease, before the strength fails. 



Bleeding, however, is not a safe remedy. As a general rule it should 

 be avoided ; and if there is any doubt as to its advisability, it is always 

 safer not to bleed. Far more horses are killed than saved by this 

 remedy. 



Bleeding is always injurious when the pulse is quick and weak. The 

 prevalent idea that a quick pulse necessarily indicates the presence of 

 inflammation, is wholly erroneous. Such a pulse far more often results 

 from a weak and perverted state of the system, which bleeding will 

 seriously, if not fatally aggravate. Bleeding is especially injurious in 

 the later stages of diseases of the lungs and air-passages, because in them 

 the patient is always weak on account of the blood not being properly 

 purified in the lungs. 



296. Sedatives. 



The term Sedatives is applied to those depressants which abate the 

 nervous force without causing positive general depression. Sedatives 

 may be divided into two classes, namely, those which possess a general, 

 and those which have only a local action. Aconite, which acts by lower- 

 ing the action of the heart, is an example of the first ; whilst warm water, 

 which assuages local pain, may be taken as a type of the latter. 



