NERVOUS TISSUE 131 



In the olfactory epithelium of vertebrates there are neuro-epithelial 

 cells which send fibers directly into the central nervous system, but in 

 other cases the nucleated bodies of the sensory cells are not found in the 

 epithelium. They occur in circumscribed masses or ganglia, from which 

 fibers extend both into the central nervous system, and outward to various 

 sensory structures, where they terminate in contact with cells which stimu- 

 late them. Thus the stimulus which gives rise to a tactile sensation is 

 received by the terminal ramifications of a nerve fiber in the skin. The 

 stimulus is conveyed along this fiber (Fig. 123, a), through the spinal 

 ganglion (b), into the spinal cord, where it produces several branches (at c). 

 One of these branches passes to a motor cell, d, to which, through contact, it 



FIG. 123. DIAGRAM OF THE SPINAL CORD SHOWING A SENSORY FIBER, a; A MOTOR FIBER, e; AND THE 

 FIBERS WHICH CONNECT THEM WITH EACH OTHER AND WITH THE BRAIN. 



transmits its stimulus. The motor cell sends a fiber outward (e) to termi- 

 nate in contact with a striated muscle, which is thereby stimulated so that 

 it contracts. This direct path from the sensory ending to the muscle, pro- 

 vides for reflex or unconscious action, such as is taken when the hand is 

 suddenly withdrawn from a painful contact. In such a case a considerable 

 group of muscles may contract together, since the sensory fiber sends 

 branches up and down the cord (/"), and these in turn give off collateral 

 branches which pass to motor cells at different levels. 



The cell which conveys the tactile sensation from the skin to the spinal 

 cord gives rise to branches which terminate in contact with other cells in 

 the spinal cord, as shown in Fig. 123, g. From these cells processes cross 

 to the opposite side of the cord and pass up to the brain (ti), where they 

 connect with nerve cells through which the sensations become conscious. 

 These brain cells presumably become permanently modified by the sensa- 

 tions which they receive, so that they store experiences. As a result of the 

 sensation transmitted from the skin, certain cells in the brain may send 

 stimuli downward to the motor cells of the cord, which then cause the 



