678 Journal of Agriculture. [10 Nov., 1908. 



associated with the internal ear, we find on each side a proprioceptive organ, 

 the labyrinth or semi-circular canal svstem. which possesses a very signal 

 importance (Fig. 56). The leading feature in each labyrinth is the pre- 

 sence of three bony tubes each curved so as to farm an arch and communi- 

 cating with a common reservoir at its base. One arch lies on the flat, that 

 is, in the horizontal plane, whilst the other two are vertical, one running 

 in a fore and aft direction and the other in a side to side direction. Within 

 these tubes is a watery fluid into which dips, in each tube, a brush of fine 

 filamental receptors. Held in suspension in the fluid are certain fine par- 

 ticles of solid matter like sand-grains. Now, in whatever position the head 

 is placed, the solid grains will gravitate in a particular direction, and if the 

 head be moved the fluid bv inertia will circulate in a definite mannex. It 

 is thus that an animal attains its knowledge of head position and move- 

 ment. If, in a pigeon, the horizontal arches are destroyed, the bird ap- 

 parently loses all .sense of movement in a horizontal plane and allows its 

 head to wag from side to side. The labyrinth also supplies the requisite 

 information for maintaining a definite posture and for preserving balance. 

 It is to the presence of the labvrinth that we may ascribe that great expan- 

 sion of the central nervous system called the cerebellum, for it is here 

 that the impulses, continually received from the labyrinth, are co-ordinated 

 and dispatched to other regions of the central ner\ous system concerned 

 with muscular innervation. 



INTEROCEPTORS. — These are found in the lining of the alimentary 

 tube and are concerned with food. In the mouth, and particularly at the 

 back of the tongue, are taste receptors. These respond to various chemical 

 bodies when in solution and give rise to the sensations of sweet, bitter, 

 saline, acid and alkaline. It must be remembered that the majority of the 

 sensations looselv called tastes are really flavours and are detected by the 

 nose which is in communication with the back of the mouth. The sensa- 

 tions of thirst, hunger and satietv are probably referable tO' receptors in 

 the mucous membrane of the alimentar\ canal from, the pharynx to the 

 stomach 



SKIN-RECEPTORS.— These give the animal cognisance of contact 

 and of the incidence of heat. Scattered irregularly over an area that in- 

 volves the entire skin and dips, for a short distance, into the mucous mem- 

 brane of the canals opening on the skin, are receptors that are sensitive to 

 ri.se of temperature, others to fall of temperature, and others to distortion 

 of the skin (touch). Manv hairs are attached at their roots to receptors 

 and owing to the leverage which thev exert can give rise to a sensation even 

 on the most delicate displacement. 



TELRRECEPTORS.— These are receptors which give cognisance ot 

 changes in the outside world at some distance from the animal. To them 

 are due the faculties of seeing, hearing and smelling. 



I. THE EYE. — This organ may be roughly likened to a photographic 

 camera. It possesses a lens system w^hich focusses a picture of the outside 

 world (when illuminated) on a sensitive surface; — the retina — in which are 

 placed receptors speciallv .sensitive to the ethereal disturbances known as 

 light. The structure of the eye is in brief detail as follows (Fig 57). In 

 front is a transparent convex sheet of tissue called the cornea which is 

 inserted, like the crvstal of a watch, into the very tough white tissue called 

 the SCLEROTIC which constitutes the major portion of the shell of the eyeball. 

 The cornea is well supplied with pain receptors and probably wdth these 

 alone. It is kept moist, and its surface smooth and free from foreign 

 matter, bv the rhvthmic wiping action of the upper eyelid, or in some 

 animals by the transverselv moving nictitating memljrane, the under surfaces 



