Physiology of Smell
The stimuli that excite the olfactory cells are certain chemical substances termed odoriferous. They must be presented in a gaseous or finely-divided form, and they reach the olfactory region in the inspired air or by diffusion. They are then dissolved in the fluid that bathes the olfactory cells, and act as chemical stimuli.
That liquids cannot act as olfactory stimuli unless they are volatile is shown by the fact that if the nasal cavities be filled with an odourous fluid such as rose-water no smell is perceived. At the same time, unless there is sufficient moisture round the hair cells to dissolve the odourous substances, the sense of smell is greatly impaired, as is evidenced in cases of dryness of the lining membrane of the interior of the nose.
The sense of smell is very delicate in man but much more so in many animals. The human olfactory organ can, for example, recognise camphor in a dilution of 1/400000, musk in a dilution of 1/800000, etc. The organ is, however, quickly fatigued for any particular odour, especially if it be a faint one; this, however, does not affect its sensitiveness to other odours.
Several attempts have been made to classify odours, but all are unsatisfactory. The best is that of Zwaardemaker, viz. :
I. Pure odours.
II. Odours mixed with sensations of common sensibility.
III. Odours mixed or confused with tastes
Pure odours are further divided into:
9. Nauseating or Foetid.
When a mixture of odours is submitted to the olfactory organ the separate odours can generally be recognised. In some cases, however, the combination of odours gives no reaction; thus if iodoform and Peruvian balsam be mixed in suitable quantities the mixture is odourless, or, alternatively, if iodoform be presented to one nostril and Peruvian balsam to the other no sensation of smell may be perceived.
Human nervous system
The Animal Cell
Nerve Cells and Nerve Fibres
General Construction and Development of the Nervous System
The Spinal Cord
The Chief Fibre Systems of the Cerebro-Spinal Axis
The Areas of Localisation on the Cerebral Cortex
The Sense Organs
Human Eye Anatomy
Human Ear Anatomy
Human Nose Anatomy
Physiology of Smell
Human Tongue Anatomy
Human Brain Anatomy
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