104 THE NERVE TISSUE 



stimuli from such sources arouse nerve impulses which are trans- 

 mitted by the neurons to others ending in effectors. Such effectors 

 are organizations of the nerve endings with other tissues which 

 they stimulate into action The eye and skin are exteroceptors 

 in that they recei^•e stimuli from outside the body. There is another 

 group of internal receptors, the enteroceptors, or visceral receptors 

 which receive stimuli from internal pressure. Still other receptors, 

 the proprioceptors, supplement the internal and external receptors 

 and lead to a regulation of the reactions set into play by the impulses 

 conveyed to the effectors. A few of the many devices of receiving 

 and affecting stimulation will serve as examples of the interrelation 

 of the neurons with other tissues. Those nerve endings receiving 

 stimulation are called sensory; those effecting stimulation of other 

 tissues or cells are the motor endings. 



Sensory Endings.— The free ending of fine nerve branches demon- 

 strated between and close to epithelial cells are associated with 

 l)oth sensory and motor impulses. Morphologically sensory and 

 motor free ends of nerves are often similar. 



In glands, the terminal ends of sympathetic fibers form a network 

 just outside the basement membrane; some branches pass through 

 this, forming another net around the bases of the gland cells, and 

 some small branches extend between the gland cells. Some of these 

 act as receptors and others as effectors regulating secretion. An 

 arrangement of free endings similar to this is present in stratified 

 squamous epithelium, as, for example, the epidermis, where it is 

 sensory. Organs of special sense have epithelial cells, derived from 

 ectoderm which are especially sensitive to particular types of stimu- 

 lus. In the upper back region of the nasal passage, among the 

 protective cells of the membrane, are special olfactory cells which 

 connect basally with nerve fibers of afferent neurons, forming part 

 of nerve pathways to the olfactory center of the brain. There are 

 special cells in the cochlea of the ear which connect with processes 

 of neurons belonging to the auditory branch of the eighth ner\'e, 

 and so form part of a pathway to the auditory center in the brain. 

 These pick up vibrations which become translated as sounds. The 

 rods and cones of the retina are stimulated by light wa\'es and 

 connect with a bipolar cell whose outer ends connect with other 

 neurons which extend in toward the brain. 



Skeletal muscles have sensory end-organs, called muscle s])indles. 

 (Fig. (JO.) They are formed about a group of small, weak, pale 

 muscle fibers, separated by connective tissue from the surrounding 



