1 66 MATER i A MEDIC A AND THERAPEUTICS. 



ACTION AND USES. 

 1. IMMEDIATE LOCAL ACTION AND USES. 



Externally. Applied for a time to the skin, dilute hydro- 

 cyanic acid causes numbness, directly depressing the sensory 

 nerves. It is used, largely diluted, to relieve itching, but must 

 not be employed where the surface is raw from scratching, as 

 it is readily absorbed by wounds. 



Internally, it produces a peculiar mixed sensation on the 

 mouth and throat, and acts as a sedative to the nerves of the 

 stomach. It is in common use to relieve pain and arrest 

 vomiting in painful dyspepsia, ulcer of the stomach, cardialgia, 

 and reflex or other nervous cases, e.g. in phthisis and 

 pregnancy. The specific action of the drug on the medulla, to 

 be presently described, doubtless assists its local effect upon 

 the gastric nerves in producing these results. 



2. ACTION ON THE BLOOD. 



Hydrocyanic acid enters the blood very rapidly from all 

 parts, especially the lungs, and produces an important change 

 on the red corpuscles. If freely given, it converts the blood of 

 the veins first into a bright arterial colour, and then into a deep 

 black, the former change arresting the oxygenating function of 

 the corpuscles, the latter destroying them. When studied in 

 drawn blood, these effects are found to be due partly to reduc- 

 tion of the oxyhaemoglobin, the oxygen being replaced by 

 cyanogen, forming cyano-haemoglobin ; and partly to union of 

 the cyanogen with oxyhaemoglobin, making cyano-oxyhaemo- 

 globin. Thus changed, blood does not give up oxygen to 

 oxydisable bodies, e.g. the guaiacum reaction cannot be ob- 

 tained. These effects of hydrocyanic acid on drawn blood 

 must not be too readily applied to the circulating fluid within 

 the body, where its action in medicinal doses is chiefly local 

 and specific. 



3. SPECIFIC ACTION AND USES. 



Hydrocyanic acid rapidly enters the tissues, and acts 

 chiefly upon the nervous structures. Considerable doses cause 

 giddiness, faintness, nausea, a constricted feeling in the 

 chest, headache, mental confusion, disturbed breathing, slow- 

 ing of the pulse, and muscular debility. Larger doses aggravate 

 these symptoms, and produce great dyspnoea and other signs of 

 nsphyxia; whilst in still larger quantity it is familiar as one of 

 the most swift and deadly of poisons. Analysis proves that 

 this drug, whilst depressing all nervous tissues, acts first 

 and chiefly upon the respiratory centre, which is briefly excited 



