THE SENSE ORGANS 89 



The seventh, or facial nerve is the nerve of the second, or hyoid 

 arch, the gill pouch in front of which is represented by the cavity 

 of the middle ear. It is distributed chiefly as a motor nerve to the 

 cutaneous muscles of the head, which are modified visceral muscles 

 from the region of the hyoid arch, but contains also taste fibres 

 from the front part of the tongue. The ninth, or glossopharyngeal 

 nerve, belonging to the third visceral (first branchial) arch, and 

 the tenth, or vagus, belonging to the fourth and succeeding visceral 

 arches in lower forms, are distributed as visceral efferent nerves 

 to the pharyngeal and laryngeal musculature, and as visceral 

 afferent nerves to various visceral organs, the ninth nerve supply- 

 ing the gustatory organs of the back part of the tongue. The vagus 

 contains a variety of fibres, both afferent and efferent, the former 

 from the larynx and respiratory organs, the latter distributed to 

 the organs of circulation and digestion. 



The eleventh, or spinal accessory nerve has apparently been 

 formed by an association of certain motor components separated 

 off from the vagus with others derived from the anterior spinal 

 nerves. The spinal elements have a characteristic distribution to 

 the cleidomastoid, sternomastoid, and trapezius muscles of the 

 side of the neck and shoulder while the other components join the 

 vagus and are distributed with it. 



The twelfth, or hypoglossal nerve has the relation of the ventral 

 or motor portion of a spinal nerve, and is distributed as a motor 

 nerve to the muscles of the tongue. 



The Sense Organs 



The complete mechanism involved in the performance of an 

 action comprises of necessity a receptor, an afferent conductor, an 

 adjustor, an efferent conductor, and an effector. The effector may 

 be a gland or a muscle, both of which have been considered in 

 previous chapters; the afferent and efferent conductors are the 

 nerve fibres which run through the peripheral nerves and are usually 

 partly within the central nervous organs; the adjustor is in the 

 spinal cord or in the brain; while the receptor is at the periphery 

 and has been defined as an organ designed to lower the threshold 

 of excitability for one kind of stimulus and to heighten it for all 

 others. 



