Volatile Oils

The most complex and perhaps the most fascinating of herbal constituents, providing the herbal practitioner with one of the more potent aids to treatment, volatile oils are nevertheless barely recognized as useful by conventional pharmacologists.

Excessive boiling of remedies or their prolonged storage in adverse conditions will lead to an inevitable loss of the volatile component, a fact, of course, that can be readily appreciated by a reduction in aroma. Quality control of aromatic herbs is thus very simple although there may be more to the aroma than that.

Generally speaking volatile oils are mixtures of hydrocarbons and oxygenated compounds derived there from. As the oxygenated form is more soluble in both water and alcohol, it is this form that on the whole determines the taste and smell of the mixture. The most common hydrocarbon is the terpene, built up by the successive accumulation of isoprene molecules (CsHs).

Notable Monoterpenes

Monoterpenes are far and away the largest group of volatile oils.

Menthol

Extracted from peppermint (Mentha x piperita) and other members of the mint family, and existing in the crystalline state at room temperature, menthol is noticeably cooling on the skin and this is accompanied by a slight local anesthetic action and followed by reflex local vasodilation. This combination of properties makes it a popular ingredient for liniments for muscular and joint pains.

It is also a powerful antiseptic and antiparasitic and in alcoholic solution it has been used for treating ringworm. It also appears to have some benefit in treating scaling from the scalp and any accompanying hair loss. When inhaled it checks nasopharyngeal catarrh and will bring welcome relief in nasal congestion: its use here, however, should be limited to the short term, as there is evidence that it has a cumulative irritating effect on the respiratory membranes that can help prolong a tendency to catarrh.

When ingested menthol is an effective carminative, as are most plant volatile oils, but it has captured particular attention for its apparent benefit in cases of colitis and bowel disease.

Camphor

Obtained naturally from the camphor plant (Cinnamomum camphora), or produced synthetically from a base of pinene isolated from turpentine, it is also likely to be found in other plants, e.g. some of the Artemisia spp., Chrysanthemum spp. and some of the Labiatae. It is closely related structurally to Borneo/ (originally 'Borneo camphor'), found notably in rosemary (Rosmarinus officinalis) and shares many of its properties.

It is locally rubefacient and anti-inflammatory, producing also a menthol-like cooling (when isolated it shares with menthol and thymol the property of being solid at room temperature) which makes it slightly anesthetic.

Internally it encourages the secretion of saliva and digestive juices, stimulates peristalsis and relaxes sphincters: it thus aids the whole digestive process. It is used as a circulatory stimulant, dilating the peripheral circulation, but it has a paradoxical action on the heart: stimulating it if failing yet dilating the coronary circulation (oil of rosemary is considered a heart tonic in Russian folk medicine).

Many of its properties can be explained, as with other volatile oils, by the reflex responses resulting from irritation of the stomach lining, but there is a direct action on the central nervous system as well: it stimulates CNS function and is an effective counter to barbiturate and morphine respiratory depression. It may under different circumstances produce feelings of exhilaration, or drowsiness and stupor. Inhaled it stimulates mucus flow and acts to cleanse a congested condition.

Thujone

Found notably in the oil of sage (Salvia officinalis), in which it makes up almost 30 per cent of the volatile component.

It is antiseptic and carminative like other terpenes. It is however relatively toxic, and although sage is very useful as a mouthwash and gargle, it can have unfortunate effects internally if given in quantity. It stimulates smooth muscle, and having also estrogenic properties is contra-indicated in pregnancy and can arrest lactation.

To what extent thujone contributes to the unusual action of sage in inhibiting perspiration is not known (there is a possible general inhibition of glandular secretions through the body). It has a potentially useful effect in mental conditions, having in moderate doses a restorative and calming effect, probably in part through a visceral antispasmodic action: in large doses, however, it provokes irritability.

Other Monoterpenes

The sesquiterpenes are the largest group of terpenes in the plant world but only a few are volatile (notably the azulenes, bisabolol and farnesene, from chamomile and yarrow); the rest exhibit very interesting activities in other ways -some 60-70 have been found with appreciable anti tumor activity, while many others are very bitter tasting and contribute to the bitters category.

Because of their relative structural simplicity, improved analytical procedures and commercial importance in the perfumery trade monoterpenes have been quite well investigated in modern times. Many of the old claims for them have been substantiated and even extended.

Some monoterpenes have fungicidal and anthelmintic effects, e.g. thymol and ascaridole, which is the major constituent of the rather toxic traditional agent for worm infestations, chenopodium oil (Chenopodium ambrosioides). Some monoterpenes also repel insects, e.g. citronellal.

Other monoterpenes act on the nervous system. The carminatives are essentially in this category, the action being spasmolytic by local reflex on the nerve endings in the gut. However, action on the nervous system is clearly more extensive than this: in-depth analysis of the action of a proprietary German preparation 'Melissengeist', extracted from the lemon balm (Melissa officinalis), showed that of its components some, e.g. citral, limonene, citronellol, citronellal and geraniol, had appreciable sedative activity, citronellal being the most potent. They were significantly effective at concentration down to 1 mg per kg of body weight, and some had observable spasmolytic action at concentrations comparable to the morphine alkaloid papaverine. Even more interesting was the clear effect demonstrated at the highest centres, as with psycho-autonomic problems such as are accompanied by symptoms of excitability, restlessness, headaches and palpitations. The otherwise extraneous information that the same monoterpenes are found concentrated in the hippocampus (in the limbic system) of the cat provides most interesting corroboration for a specifically central action for this group of substances.

There is a further group of monoterpenes, the iridoids and their glycosides, which has a high level of pharmacological activity, but it is not volatile.

Sesquiterpenes

The sesquiterpenes, having a greater molecular weight, are much less volatile than the monoterpenes and are thus less associated with the volatile oils. The best examples of some that are come from the chamomiles (Anthemis nobilis and Matricaria recutita) and yarrow (Achillea millefolium).

The Azulenes

These substances, for example chamazulene from chamomile and yarrow and guaiazulene from lignum-vitae (Guaiacum officinale), do not exist in any quantity in the natural state. They are produced as artefacts of the steam distillation process of extraction from volatile precursors in the plant such as matricin, achillin or artabsin. They will also be found in appreciable quantity after the normal process of infusion in hot water, especially if this is done in an enclosed container that allows steam rising to recondense and drain back into the liquid: to gain the full value of remedies such as chamomile and yarrow it is necessary therefore to make a hot infusion or tisane of them, and to take a measure of care in the process.

The azulenes are effective anti-inflammatory and antispasmodic agents, reducing histamine-induced tissue reactions and calming the nervous system, both peripherally, as in visceral tension, and centrally, as in anxiety, nervous tension and headaches. Their activity also extends to reducing the anaphylaxis consequent on the allergic response and they are thus indicated for conditions such as hay fever, allergic asthma and allergic eczema. They are strongly antiseptic on contact.

Bisabolol

This is a naturally occurring constituent of the volatile oil of chamomile (along with another anti-inflammatory sesquiterpene farnesene) and acts as partial compensation for any use of the plant that does not involve heating with water. It has been shown to reduce the amount of the proteolytic enzyme pepsin secreted by the stomach without any change occurring in the amount of stomach acid. This suggests a specific interaction with pepsin activity, a possibility with great implication for the treatment of gastric and upper intestinal diseases. It has also exhibited direct anti-inflammatory action.