532 PRINCIPLES OF GENERAL PHYSIOLOGY 



evolution. Periodic impulses, if intense, may affect touch receptors without 

 causing a sensation of sound, which requires an appropriate cerebral aiiulv^T. 



There are certain organs, " statocysts," present in practically all animals from 

 the jelly-fish upwards, and indeed there are similar structures in the higher plants, 

 whose function it is to enable their possessors to appreciate their position with 

 respect to the direction of gravity. This is done by the presence of a loose 

 particle or particles in a sac, which press upon different receptor endings 

 according to the position of the sac. By introduction of iron filings, an animal 

 can be made sensitive to the direction of magnetic force. 



In addition to statocyst organs, the vertebrate possesses a system of three 

 canals, the semicircular canals, or labyrinth, on each side. These are arranged 

 to correspond with the three dimensions of space and are capable of detecting 

 rapid movements and appreciating their direction. This is done by the aid of 

 the inertia and internal friction of the liquid filling them. Sensitive hairs, attached 

 to receptor cells on the walls, are drawn through the liquid and bent, owing to 

 the fact that the liquid does not immediately follow the movement of the walls 

 of the tube containing it. 



Sensations derived from the labyrinth play a large part in the maintenance 

 of the tonic contraction of the muscles necessary for posture. 



Attention is called to the manner in which combinations of sensations from 

 different receptors are used for the purpose of forming complex notions, such as 

 those of space and time, etc. 



Plants, also, have developed means of intensifying and determining the 

 direction of external forces, especially those of gravity and of light. The region 

 of a growing root, for example, sensitive to gravity is not identical with that in 

 which the response takes place. The mechanism appears to be the same as that 

 of the animal statocysts, grains of starch being generally the movable particles. 



The direction of light is appreciated by the leaves of plants owing to refraction 

 by the outer ends of the epidermic cells, these ends being shaped as lenses. By 

 this means the spot most brightly illuminated on the base of the cell differs in 

 position according to the direction from which light rays enter the epidermis. 



Certain plants possess structures very sensitive to touch, and in many cases a 

 rapid movement of an organ results from a slight stimulus. 



LITERATURE 



Primitive Receptors. 



Parker (1903, 1905, and 1912). 



Taste. 



Nagel (1905 



Smell. 



Nagel (1905). Zwaardemaker (1902). 



Skin Receptors. 



Von Frey (1913). Head and Rivers (1908). 



Photo receptors. 



Helmholtz (1909-1910). Edridge-Green (1909, 1911). 



Von Rohr (1909) re Dioptrics of the Eye. 



Auditory Organs. 



Helmholtz (1863). Yoshii (1909). 



Statocysts. 



Von Buddenbroek (1911). Mangold (1912). 



Labyrinth. 



Ewald (1892). Wilson and Pike (1912). 



Plant Receptors. 



Haberlandt (1904, 1909). 



