lo Nov., 1908.] Elements of Aiiinial Physiology. 683 



canalis cochlece and set the fluid in tliis receptacle moving. Tiie basilar 

 membrane is affected by the movement and so causes a distortion of the 

 organ of Corti and this causes stimulation of the sound-receptors. 



The intimate connection that exists between the organ of hearing and 

 the semi-circular canal system must serve some purpose but what it is has 

 not as yet been discovered. 



III. SMELL. The sense of smell is in so many ways connected with 

 the selection of food that it is often classed with interoceptive sensations ; 

 but with the majority of animals other activities are guided by it. Thus 

 the presence of enemies can be revealed ; offspring can be recognised, 

 and traced if strayed ; vitiated air can often be recognised as 

 such ; whilst in the sexual life of most animals it plays a very important 

 part. In man the delicacy of the sense of smell is greatly inferior to that 

 existing in most other mammals, a decadence that may be due to the elevation 

 of the head through the assumption of the erect posture. Yet even as it 

 exists in man the sense of smell is many times more delicate, as a quali- 

 tative test, than the most refined methods of spectrum analysis. The recep- 

 tors for smell are placed in a small yellowish patch of mucous membrane in 

 each nasal passage. The fine afferent nerve fibres pass through the ethmoid 

 bone into the skull and enter a special protrusion of the central nervous 

 system called the olfactory bulb. The so-called olfactory " nerve " on 

 each side (first cranial^ is, like the optic nerve, a strand of white matter 

 belonging to the brain stem. Substances which are perceived by the sense 

 of smell are carried in the air in the form of gases, more rarely as matter in 

 a finely divided state, and are first dissolved bv the moisture co^•ea:ing the 

 receptor patch of mucous membrane before the sensation is evoked. 



PAIN RECEPTORS.— It is a matter of doubt whether a definite 

 apparatus is necessary to start nerve impulses classified as painful. Some 

 physiologists consider that a naked nerve-ending is sufficient. The signifi- 

 cance of pain is that, at the spot where the impulses are inaugurated, the 

 integrity of the living tissue is threatened by some destructive agency, such 

 as extreme heat, cold or pressure, wounds, chemical corrosives, &c., on the 

 skin, distension in the gut and heart, and inflammation in every region en- 

 dowed with pain nerves. The receptors, or nerve endings, are constructed 

 so as to respond to the change before the tissue is seriously impaired ; the 

 attention can therefore be directed to the abnormal part and an effort be 

 made to remove the threatening agent, or to avoid further injury. Pain 

 acts therefore like an alarm mechanism giving urgent warning that danger 

 threatens and is indeed a most beneficent provision of nature. The skin 

 and cornea are verv rich in pain receptors which are frequently in action ; 

 the alimentary and muscular receptors are occasionally active, whilst those 

 in the heart and other viscera may never start an impulse throughout a 

 whole life-time. The regions of the body devoid of pain receptors are 

 very few in number ; those that mav be mentioned are the outer layer of 

 the skin, cartilage, the retina, and the central parts of the liver. 



Visceral sensations not classifiable under the above headings arise in the 

 urinary, genital and pulmonary organs. Thev are concerned with special 

 functions of these organs when such are under anv voluntary control. 



