508 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



motor influence, which is generated in respondence to the stimulus thus 

 conveyed, appears to act chiefly through the branches of the Par Vagum, 

 which are distributed to most of the muscles concerned in swallowing ; 

 but the Facial, the Hypoglossal, the motor portion of the Fifth, and 

 perhaps also the motor portions of some of the Cervical nerves, are also 

 concerned in the movement, and may effect it, though with difficulty, 

 after the pharyngeal branches of the Par Vagum have been divided. 



898. In the propulsion of the food down the (Esophagus, to which 

 the glosso-pharyngeal nerve does not extend, the muscular contraction, 

 so far as it is of a reflex nature ( 455), must depend upon the oesopha- 

 geal branches of the Par Vagum alone ; their afferent portion being 

 the excitor, and their motor portion giving the requisite stimulus to the 

 muscles. The same must be the case in regard to the muscular contrac- 

 tions of the cardiac and pyloric sphincters, and of the walls of the sto- 

 mach, so far as regards their dependence upon the nervous system at 

 all ; but the degree of this is doubtful. 



899. There are other reflex actions of the Medulla Oblongata, con- 

 nected with the regulation of the aperture of the Glottis ; these, which 

 are effected through the superior and inferior laryngeal branches of the 

 Par Vagum, will be better noticed, when the actions of the Larynx are 

 under consideration ( 976). In like manner, the reflex action concerned 

 in the regulation of the aperture of the Pupil, will be more conveniently 

 noticed in the sketch to be hereafter given of the Physiology of Vision 

 (909). 



5. Functions of the Sensory Ganglia. 



900. All the nerves of Sensation, both general and special, may be 

 traced into a series of ganglionic masses lying at the base of the brain ; 

 which seem to constitute their awn particular centres. Thus we have 

 seen in Fishes, the Olfactive, Optic, and Auditory ganglia, marked out 

 as such, by the termination of the nerves proceeding from the organs of 

 smell, sight, and hearing, in these masses respectively. These ganglia 

 bear an evident correspondence with the cephalic ganglia of the Inverte- 

 brata ; which must chiefly, however, be regarded as optic ganglia, since 

 the development of the eyes far surpasses that of the other organs of 

 special sense. On the other hand, they find their representatives in 

 certain organs at the base of the brain, in Man and the higher Mamma- 

 lia ; which, though small in proportion to the whole Encephalon, are 

 capable of being clearly marked out as the ganglionic centres of the 

 several nerves of sense. Thus, anteriorly, we have the Olfactive gan- 

 glia, in what are commonly termed the bulbous expansions of the Olfac- 

 tive nerve ; which, however, are real ganglia, containing gray or vesicular 

 substance ; and their separation from the general mass of the Encepha- 

 lon, by the peduncles or footstalks commonly termed the trunks of the 

 olfactory nerves, finds its analogy in many species of Fish ( 869). The 

 ganglionic nature of these masses is more evident in many of the lower 

 Mammalia, in which the organ of smell is highly developed, than it is 

 in Man, whose olfactive powers are comparatively moderate. At some 

 distance behind these, we have the representatives of the Optic Ganglia, 



