228 



ELEMENTARY BIOLOGY 



FIG. 91. Simple tissues in a simple animal 



The Hydra is among the simplest of many-celled 

 animals, consisting of a hollow bag whose wall is 

 made up of two layers of cells. There is apparent 

 a division of labor between the inner layer of 

 digesting cells and the outer layer of protecting 

 cells. In a section of the wall we may see that 

 the outer cells, a, have elongations, t>, at their 

 bases, which are highly contractile, and that in- 

 terspersed among these cells are smaller ones, c, 

 which are highly sensitive and extended into 

 delicate threads and expansions, d, which may 

 be considered to correspond to nerves 



specialized chemical sense, 

 and is more highly devel- 

 oped in some of the lower 

 animals than it is in man. 

 The sneeze reflex is 

 started either by a strong 

 odor stimulation or by a 

 touch stimulation in some 

 of the nerve endings of 

 the nostrils. Watering of 

 the mouth in response to 

 certain odors illustrates 

 reflexes that are dis- 

 charged to glands rather 

 than to muscles. The feel- 

 ing of nausea and the act 

 of vomiting are reflexes 

 that may be started by stimulation of the odor end-organs. 

 267. Stimulation and sensation. In the case of touch, taste, 

 smell, and other senses, the application of energy or of contact 

 to the nerve endings (or end 

 organs) sets up a disturbance 

 in the protoplasm of the neu- 

 ron. This disturbance is not in 

 itself a sensation. The disturb- 

 ance is carried along through 

 the neuron and is passed on 

 through one or more other neu- 

 rons until it finally sets up a 

 disturbance of one or more cells 

 of the brain. It is here that the 

 stimulus is at last translated 

 into a feeling, or sensation. 



FIG. 92. The touch organs of the skin 



We perceive touch or heat or cold according 

 to the end organ that is stimulated. These 

 end organs, d, lie beneath the epidermis, , 

 and contain endings of nerve fibers, e ; b, the 

 dermis, or true skin ; c, blood vessels 



