ii6 VETERINARY PHYSIOLOGY 



through the raucous membrane, and through the cribriform 

 plate of the ethmoid into the olfactory bulb /. 



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

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

 olfactory tracts. 



2. Connections with the Central Nervous System. The con- 

 nections of the olfactory fibres with the cortical centre in the 

 cerebrum (see p. 205) is shown in fig. 51, ///. 



3. Physiology. Stimulation of Mechanism. To act upon, 

 the olfactory mechanism the substance must be volatile, and 

 must be suspended in the air. In this condition infinitesimal 

 quantities of such substances as musk are capable of producing 

 powerful sensations. The mucous membrane must be moist, 

 and this is secured by the activity of Bowman's glands, 

 situated in the mucous membrane. These are under the 

 control of the fifth cranial nerve, and section of this leads 

 indirectly to loss of the sense of smell through dryness of 

 the membrane. 



(b) FOR VIBRATION OF ETHER 



Sense of Sight 

 A. General Considerations 



While the addition to and withdrawal from the surface of 

 the body of the slower ethereal waves which are the basis of 

 heat act upon the special nerve terminations in the skin to 

 give rise to sensations of heat and cold, a certain range of 

 more rapid vibrations act specially upon the nerve endings in 

 the eye to produce molecular changes which in turn affect the 

 centres in the brain, and play a most important part in the 

 adjustment of movements for the benefit of the body and which 

 give rise to changes in consciousness which we call sight. The 

 range of vibrations which can act in this way are comparatively 

 limited, the slowest being about 435 billions per second, the 

 most rapid about 764 billions. Vibrations more rapid than 

 this, which are capable of setting up chemical changes, as in 

 photography, do not produce visual sensations. 



The action of light upon the protoplasm of lower organisms 



