564 A MANUAL OF VETERINARY PHYSIOLOGY 



stimuli. Tracing these nerves backwards into the brain, they 

 are found to terminate in globular bodies in the olfactory lobes, 

 where they connect with the special cells of this region through 

 the dendrites of the latter. The axons from the new cells are 

 continued into the olfactory tracts of the brain by means of 

 bundles of fibres, some of which terminate in the tract ; others 

 cross to the opposite side, to end in the grey matter of the 

 hippocampal lobes ; others pass to the hippocampal lobes of 

 the same side. The sense of smell is located in this region, 

 but the connections of the olfactory tract are numerous and 

 incompletely known. The olfactory area is interesting de- 

 velopmentally, as being the first part of the brain to put in an 

 appearance, showing the extraordinary importance of the sense 

 of smell in animals very low in the scale. In animals possessing 

 acute sense of smell the olfactory regions are highly developed 

 in others, like man, where additional protective instincts have 

 subsequently appeared, the sense of smell is not so acute, and 

 the central representation accordingly greatly reduced. In 

 animals where the sense of smell is lost, as in the porpoise and 

 most fishes, the olfactory region is absent, while in others, like 

 the shark, which is guided entirely by this sense, its development 

 is extraordinary. In a shark 25 feet long the olfactory nerves 

 spread out over an area of 12 to 13 square feet (Halliburton). 



The Olfactory Stimulus. — Before an odour can affect the 

 olfactory nerves it has to diffuse into the higher cavities of the 

 nasal chambers, and from being gaseous it must become dissolved 

 in the fluid which bathes these surfaces, for a dry olfactory 

 surface is insensible to smell. We have no idea of the nature 

 of the particles which constitute an odour, but it is supposed 

 that they act chemically on the hair cells. The odour of a body 

 can be detected with greater accuracy by ■ sniffing ' ; by this 

 inspiratory act no time is lost in diffusion occurring between the 

 respiratory and the olfactory region, as the odoriferous particles 

 are forcibly drawn upwards. Colin performed a tracheotomy in 

 horses, and tied the trachea above the incision. Under these 

 conditions no air could pass up into the nasal chambers excepting 

 by diffusion. Animals so treated were unable, with their eyes 

 covered, to detect the presence of hay or oats. A stallion, how- 

 ever, under these conditions, was able to recognise a mare, 

 though he failed to recognise food. 



Olfactory Sensations. — There are certain odours which excite 

 the olfactory organs more readily than others ; thus flesh, blood, 

 and offal have a remarkably stimulating effect on the carnivora, 

 whilst grass, grain, and vegetable products generally, stimulate 

 the herbivora. The odour of blood or flesh is evidently repulsive 

 to the herbivora, and may even cause nervousness and fright ; 



