326 FUNCTIONS OF THE NERVOUS SYSTEM. 



direction. The three pairs of nerves of special sensation make their way 

 out through these three cranial vertebrae respectively. At a later period of 

 development, other nerves are interposed between these ; which, being inter- 

 vertebral, are evidently more analogous to the Spinal nerves, both in situation 

 and function. A separation of the primitive fibres of these takes place, how- 

 ever, during the progress of development, so that their distribution appears 

 irregular. Thus the greater part of the sensory fibres are contained in the 

 large division of the Trigeminus ; whilst, of the motor fibres, the anterior 

 ones chiefly pass forwards as the Oculo-motor and Patheticus ; and of the 

 posterior, some form the small division of the Trigeminus, and others unite 

 with the first pair from the Medulla Oblongata to form the Facial. This last 

 fact explains the close union which is found in Fishes and some Amphibia, 

 between that nerve and those proceeding more directly from the Medulla Ob- 

 longata. According to Valentin, the Glosso-pharyngeal is the sensory por- 

 tion of the first pair from the Medulla Oblongata, of which the motor part is 

 chiefly comprehended in the Facial nerve. It is very interesting to trace this 

 gradual metamorphosis from the character of the Spinal nerves, which is ex- 

 hibited in the Cephalic, when they are traced upwards from the Medulla Ob- 

 longata ; and this is shown also, in some degree, in the nerves of special sen- 

 sation ( 446, or). Although we are accustomed to consider the Fifth pair as 

 par eminence the Spinal nerve of the head, the foregoing statements, founded 

 upon the history of development, show that the nerves of the Orbit really 

 belong to its motor portion ; they may consequently be regarded as altogether 

 forming \\\e first of the inter vertebral or Spinal nerves of the cranium. The 

 Facial and Glosso-pharyngeal appear to constitute the second ; whilst the Par 

 Vagum and Spinal Accessory, forming the third pair, intervene between this 

 and the true spinal, of which the Hypoglossal may be considered as the first. 



5. Of the Sensory Ganglia and their Functions. Consensual Movements. 



422. At the base of the Brain in Man, concealed by the Cerebral Hemi- 

 spheres, but still readily distinguishable from them, we find a series of gan- 

 glionic masses ; which are in direct connection with the nerves of Sensation; 

 and which appear to have functions quite independent of those of the other 

 components of the Encephalon. Thus 'anteriorly we have the Olfactive 

 ganglia, in what are commonly termed the bulbous expansions of the Olfactive 

 nerve. That these are real ganglia, is proved by their containing grey or ve- 

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

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

 tory nerves, finds its analogy in many species of Fish ( 357). 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 olfac- 

 tive powers are comparatively moderate. At some distance behind these, we 

 have the representatives of the Optic ganglia, in the Tubercula Quadrigemina, 

 to which the principal part of the roots of the Optic nerve may be traced. 

 Although these bodies are so small in Mail, in comparison to the whole En- 

 cephalic mass, as to be apparently insignificant, yet they are much larger, and 

 form a more evidently important part of it in many of the lower Mammalia; 

 though slill presenting the same general aspect. The Auditory ganglia do 

 not form distinct lobes or projections ; but are lodged in the substance of the 

 Medulla Oblongata. Their real character is most evident in certain Fishes, as the 

 Carp ; in which we trace the Auditory nerve into a ganglionic centre as distinct 

 ns the Optic ganglion. In higher animals, however, and in Man, we are able 

 to trace the Auditory nerve into a small mass of vesicular matter, which lies 

 on each side of the Fourth Ventricle ; and although this is lodged in the midst 



