THE NERVOUS SYSTEM 



189 



muscle contracts, the muscle spindle is stimulated and produces 

 eventually the sensation of movement. In other words, the 

 muscle and tendon spindles are the terminations of sensory nerves 

 and the motor end plates of motor nerves (Figs. 121 and 122). 



Taste Receptors. — A sense of taste is probably present in all 

 animals, but since the end organs of both taste and smell are 

 activated by chemical stimuli, it is often difficult in lower animals 

 to differentiate between them. Thus, if a piece of crushed clam 

 be placed near a burrow of Nereis virens, a marine annelid, the 

 animal responds by thrusting its anterior 

 end out of the burrow toward the food 

 which it seizes, pulls back into the 

 burrow, and devours. The animal is 

 evidently stimulated by the meat juices, 

 but whether by taste or smell or a com- 

 bination of both, it is difficult to decide. 

 Insects are provided with organs of taste 

 located on the mouth parts and the 

 antennae and in some cases at least on 

 the legs. The taste receptors of verte- 

 brates are known as taste buds (Fig. 123). 

 Each consists of a number of taste cells 

 provided with short bristles at their outer 

 ends and a number of supporting cells, 

 the whole forming a spherical body 

 buried in the surrounding tissue except 

 for a small area at the surface containing 

 a pore. The outer ends of the taste cells 

 converge at the pore. In fishes taste buds occur in the walls of 

 the pharynx, on the gills, on the outside of the body, and in some 

 (catfish) in the tail and the barbels. In higher vertebrates they 

 are restricted to the oral cavity, where they are found in the 

 tongue, soft palate, and epiglottis. The base of each taste cell 

 is in contact with a nerve ending from which the sensory impulse 

 is conveyed to the brain. In mammals the nerves involved in 

 the sense of taste are a branch (chorda tympani) of the seventh 

 cranial nerve and the lingual branch of the ninth. 



Smell Receptors. — Olfactory organs are stimulated by chemical 

 action, but the amount of the stimulating agent required is very 

 much less than in the case of the taste receptors. Smell has been 



Fig 



121. — Pacinian corpus- 

 cle, diagrammatic. 



