THE SENSE ORGANS 



571 



bottom of the groove become differentiated into clusters of neuromast 

 cells surrounded by indifferent supporting cells. Mucus cells also develop, 

 and fill the canal with their secretions. Nervous connexions are second- 

 ary, and are formed by neurites from neural crest cells, usually of the 

 vagus and facial ganglia, but sometimes of the glossopharyngeal ganglion. 



MUSCLE SPINDLES 



Muscle spindles form a special set of sensory nerve terminations 

 upon muscle fibers, by means of which we are able to sense the degree of 

 contraction of a muscle and the position of parts of the body. Such 

 neuromuscular spindles are found in vertebrates, beginning with the 

 Amphibia. As their name suggests, they are spindle-shaped structures 

 located among the fibers of a muscle, usually near where the muscle 

 joins a tendon. 



MUSCLE FIBERS 



N 



DENDRITES RINGS 



SPIRALS SHEATH 



-.-.--/V-U.— ^ 



.. .... • /• -.v. -1 ■' -.■=?.< 



^■■" "*" 



SHEATH 



Fig. 470.^A portion of a muscle spindle from a gold chloride preparation of a cat 

 muscle. The sensory nerve terminations take the form of spirals, or rings, or branched 

 dendrites. A connective tissue sheath surrounds the spindle. (Redrawn from Jordan 

 and Ferguson, after Ruffini.) 



Each spindle usually consists of several striped muscle fibers attached 

 to a tendon at one end and to intramuscular connective tissue at the other. 

 One or more nerve fibers connect with the spindle. Each nerve fiber 

 terminates within the spindle in the form of a spiral encircling a muscle 

 fiber. 



Neurotendinous spindles resemble muscular, but are connected with 

 tendons. These also serve to indicate the amount of muscular work and 

 the resistance overcome. 



OLFACTORY ORGANS 



There are two chemical senses, smell and taste, but distinction between 

 the two is difficult to draw in lower animals in which differentiated organs 

 are wanting. For smell and taste alike chemical substances, in order to 

 affect sense receptors, must be dissolved in water. This response to dis- 

 solved chemicals is a fundamental property of organisms, as is shown by 

 such facts as that an amoeba will engulf a protein particle but not a piece 



