134 



VETERINARY PHYSIOLOGY 



terrestrial animals by volatile substances inhaled through 

 the nose, and in fishes by substances dissolved in water. 

 Aquatic mammals, such as the Cetacea, possess this mechanism 

 in an imperfectly developed condition, and seem to rely on 

 the sense of taste. They have been called anosmatic. 



In the dog, and in many other mammals, this mechanism 

 is enormously more developed than in man, and it plays a 

 much more important part in the lives of these animals. 



A. Receptors. — Over the upper part of the nasal cavity 

 the columnar epithelial cells are devoid of cilia, and between 



He. 



T.N. 



Fig. 60. — Some of the more important Central Connections of the Olfactory 

 Receptors. O.F., olfactory cell ; O.B., olfactorj- bulb ; O.T., olfactory 

 tubercle; U., uncus; He, hippocampus; M., corpus mammillare : 

 T.A., thalamus; /ffe., habenula ; T.X., tegmental nucleus; T., fibres 

 to the tegmentum. (Bryce.) 



them are placed spindle-shaped cells (fig. 59, A), which 

 send processes through the mucous membrane, and through 

 the cribriform plate of the ethmoid into the olfactory bulb I. 



In the bulb these neurons form synapses, B, with other 

 neurons, C, the axons of which pass to the base of the 

 olfactory tracts (fig. 60). 



B. Connections vi^ith the Central Nervous System. 

 — The olfactory tract connects with ((t) the olfactory 

 tubercle which is rudimentary in the human brain, and with 

 (6) the pyriform area (fig. 61), which forms a prominent 

 feature on the base of the brain of animals in which smell 

 plays an important part, but which is less developed in the 



