200 HISTOLOGY 



LITERATURE 



HARDESTY, I. "The Neuroglia of the Spinal Cord of the Elephant," Am. Journ. of 



Aiiat., 1902-1903, Vol. II, p. 81. 

 HUBER, G. C. "Studies on Neuroglia Tissue," etc., Contrib. Med. Research, ded. to 



V. C. Vaughan, Ann Arbor, 1903, p. 578. 



JOSEPH, G. "Zur Kentniss der Neuroglia," Anat. Anz., Band XVII. 

 WAWRZIK, E. "Uber das Stutzgewebe des Nervensystems der Chaetopoden," Zool. 



Beitrage, Band III, 1892. 

 HATAI, SHINKISHI. " On the Origin of Neuroglia Tissue from the Mesoblast," Journ. 



Comp. Neurol., Vol. XII, No. 4, 1900. 



TISSUES OF TOUCH, OR TACTILE TISSUES 



The simplest form of stimulus that can be perceived by the cells of the 

 body is probably some form of motion, the movements of some thing, be it 

 a solid, a fluid, or a gas. There are many kinds of such movements, from 

 the slow pressure of a rigid mass up to the rapid motions of waves of the 

 atmosphere or other gases. Also there are certain qualities of these 

 movements, such as direction, duplication, repetition, and rhythm that 

 can be perceived by some cells and not by others. 



The cells which can only perceive the movements of bodies in a gen- 

 eral way, including pressure and impact, are known as the cells of touch 

 or tactile cells. Those which make use of the impact or pressure of special 

 bodies under the influence of gravity or spatial relations to determine 

 the position of the body are the cells of equilibration or static cells. Those 

 which perceive the motion of the air waves, either directly or as trans- 

 mitted and represented through the mechanical vibrations of substances 

 affected by these waves, are called the sense cells of hearing or the auditory 

 cells. These several kinds of nervous tissue, all alike, perceive only the 

 mechanical movements of matter. 



It is characteristic of these tissues that the perceptory cell itself almost 

 never receives the stimulus first hand, but usually from some other and 

 intervening tissue cell or dead cell, cell-product or foreign body that, 

 while itself not sensitive, is yet able to convey the motion stimulus to the 

 real perceptory cell by transmitting, sometimes with modifications, the 

 movement through its own mass. We shall caJJ such bodies the inter- 

 mediate tissues or substances. These intermediate cells and materials 

 form organic parts of the sensory apparatus, many of which would not 

 be able to operate without them or would send in exaggerated tactile 

 or pain sensations, or none at all. 



Some few forms of tactile cells, found in various groups of animals, 

 receive the stimulus almost directly upon their sensory surfaces and 

 practically without the aid of intermediate cells or tissues. Such a case 



