Part 4: A Dog’s Nose (Section 2)

Wednesday, January 31, 2018 6:32 PM | Front & Finish (Administrator)

Written by Michael Pumilia

A longitudinal cross section of the average dog’s nose is shown in figure 4-9. It is a rendering based on Magnetic Resonance Imaging (MRI). The use of MRI’s has really increased our understanding of all things within animals of all types. In the figure, the cross section of a dog’s nose is shown, with the tip of the nose on the left hand of the artwork. Thus we are looking at the inner workings of the right side of the head. “The internal fluid dynamics of olfaction in the dog is complicated by the compact, multipurpose design of the nasal cavity, where chemical sensing and respiratory air conditioning both occur. Computational solutions of inspiratory airflow during sniffing show that, although combined within the same organ, olfactory and respiratory airflows are fundamentally separate phenomena, each with a distinct flow path through the nasal cavity. In detail (a): Unsteady pathlines generated from trajectories of neutrally buoyant particles released from the naris (dog’s nostril) at equally spaced time intervals throughout inspiration reveal distinct respiratory and olfactory flow paths within the nasal cavity. In detail (b): The same inspiratory pathlines coloured by velocity magnitude show high velocity olfactory airflow travelling back through the dorsal meatus and low velocity airflow filtering through the olfactory recess in the forward–lateral direction. In detail (c): Expiratory pathlines originating from the nasopharynx (see figure 4-8) demonstrate that airflow bypasses the olfactory recess during expiration, leaving quiescent scent-laden air there, providing an additional residence time for enhanced odorant absorption.

Figure 4-9. The Intranasal Fluid Dynamics of Canine Olfaction.
[In detail (a) Red lines are olfactory pathlines; Blue lines, respiratory pathlines]

The anatomical structure of the canine nasal cavity is remarkably well organized for efficient intranasal odorant transport, which may partly explain macrosmia [explained below] in the dog and other similarly organized animals. The overall location and configuration of the sensory region is shown here to be critical to the intranasal fluid dynamics of canine olfaction, forcing a unique nasal airflow pattern during sniffing that is optimized for odorant delivery to the sensory part of the nose.

Specifically, the relegation of olfaction to an olfactory recess, in the rear of the nasal cavity and off the main respiratory passage, forces unidirectional airflow there during inspiration and a stagnant period during expiration.” (Reference 1)

            It’s the inner structure of the nose that provides the pathways for the scent leaden air to travel through the respiratory and olfactory epithelium of the dog. The cross section of the olfactory recess in figure 4-10 shows the scroll work of tissue where the olfactory receptors are located.

Figure 4-10. Computer Model of the Left Side of the Canine Nasal Airway    

The olfactory epithelium, which exclusively contains Olfactory Receptor Neurons (ORNs), is confined to the ethmoidal region (olfactory region) of the nasal cavity, where it lines the bony scrolls known as ethmoturbinates. Figure 4-11 shows a better illustration of the scroll work in the respiratory and olfactory regions. In the details of the figure, the white spaces are the air passages.

Figure 4-11. The mesh resolution in the respiratory (left) and olfactory (right) regions of the nasal cavity are shown. 

The respiratory region as shown in the detail in figure 4-11 has much more open spaces than the olfactory region because its purpose is to condition the air entering the nose. This area provides the heating and humidification of the inhaled air. This improves the air so that it can pass through the maxilloturbinate airway (see figure 4-8) toward the nasopharynx, where it enters the lower respiratory tract, completely bypassing the olfactory recess.

The olfactory region contains a large surface area for odorant absorption to enhance ‘‘chromatographic’’ separation patterns, which may aid in odor discrimination. The surface consists of the epithelium where the ORN cells are covered with cilia (small whiskers) and then a layer of stagnant mucus. The odor molecules diffuse through the mucus until they reach the ORN binding sites. Beneath these sites are free nerve endings which convert the chemical signals into electrical signals that are sent to the brain. There the signals are processed. Airflow, shown in figure 4-12, enters the nose at the left through the nasal vestibule. The red, dark blue, and green streamlines illustrate the dorsal, lateral, and ventral olfactory flow paths, respectively. At the aft dorsal meatus the same inhaled air is divided into three streams which are directed to different areas within the olfactory recess. “The most significant distinction between the various olfactory streamlines is their residence time in the olfactory recess. In general, olfactory airflow that passes through the dorsal ethmoturbinates resides in the nasal cavity significantly longer than olfactory flow that passes through the ventral or lateral ethmoturbinate regions. Across all flow rates, olfactory airflow comprises approximately 15% of the total airflow inspired by the dog, the remainder going toward respiration.”  (Reference 4) This separate processing of the same air is the definition of chromatographic separation.

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