90 ANATOMY FOB, NUESES. [CHAP. VII. 



of the pharynx, of motion to the pharyngeal muscles, and the special nerve 

 of taste to part of the tongue. 



The pneumogastric nerve has a more extensive distribution than any of 

 the other cranial nerves, passing through the neck and thorax to the upper 

 part of the abdomen. It contains both motor and sensory fibres. It sup- 

 plies the organs of voice and respiration with motor and sensory filaments ; 

 and the pharynx, oesophagus, stomach, and heart with motor fibres. 



The spinal-accessory nerve consists of two parts : one, the spinal portion, 

 and the other, the accessory portion to the tenth nerve. It is a motor nerve 

 supplying certain muscles of the neck. It differs from the other cranial 

 nerves in arising from the spinal cord, but it leaves the skull by the same 

 aperture as the pneumogastric and glosso-pharyngeal. 



The hypoglossal nerve is the motor nerve of the tongue. 



It will be observed that of the twelve pairs of cranial nerves, four and a 

 part of a fifth, are distributed to the eye, viz. the optic, motor occuli, pa- 

 thetic, abducens, and the ophthalmic branch of the fifth. The ear has one 

 special nerve, the auditory, and is sparingly supplied with motor and sensory 

 fibres from other nerves. The nose has also one special nerve, the olfac- 

 tory, and is more abundantly supplied than the ear, with motor and sensory 

 fibres from other nerves. The tongue has two special branch nerves of taste, 

 the lingual, a branch of the fifth, and the glossal, a branch of the ninth ; 

 it has also its own motor nerve, the hypoglossal. 



The physiology of the nervous system. The physiology of the 

 nervous system, though exceedingly complex in its details, is, 

 in its essentials, not difficult to understand. 



The simplest nervous mechanism is the reflex arc, and the 

 simplest form of nervous activity is "reflex action." Two 

 neurones enter into the formation of a reflex arc, a sensory 

 neurone and a motor neurone. On applying an appropriate 

 stimulus to the peripheral end of the sensory neurone an im- 

 pulse is generated which passes along the sensory neurone to 

 the nerve centre, and back again to the periphery by the motor 

 neurone ; and, since the motor neurone terminates in a muscle 

 (or some similar mechanism), we get a muscular response as 

 the indirect result of stimulating the sensory nerve. 



The kind of stimulus which will call forth the nerve impulse 

 depends on the peripheral termination of the sensory nerve, and 

 the kind of response which an appropriate stimulus will call 

 forth depends on the mode of termination of the motor nerve. 

 Thus light falling on the retinal coat of the eye (the peripheral 

 termination of the sensory nerve) generates an impulse which 

 passes to the centre by the optic nerve, and returns again by 

 the oculomotor nerve to the periphery, the sphincter of the 



