182 FUNCTIONS OF THE NERVOUS SYSTEM. 



therefore, we proceed to the rest. Comparative anatomy, and the study of 

 Embryonic development, alike show that the Spinal Cord and Medulla Ob- 

 longata constitute the most essential part of the nervous system in Vertebrata ; 

 and that the Cerebral Hemispheres are superadded, as it were, to this. At 

 an early period of development, the Encephalon consists chiefly of three 

 vesicles, which correspond with the gang-home enlargements of the nervous 

 cord of the Articulata, and mark three divisions of the cerebro-spinal axis ; 

 and, in accordance with this view, the Osteologist is able to trace, in the bones 

 of the cranium, the same elements which would form three vertebras, in a 

 much expanded and altered condition. However improbable such an idea 

 might seem, when the cranium of the higher Vertebrata alone is examined, it 

 at once reconciles itself to our reason, when we direct our attention to that of 

 Reptiles and Fishes ; in which classes the size of the Cerebral or hemispheric 

 ganglia is very small, in comparison with that of the Ganglia of special sensa- 

 tion ; and in which the latter evidently form but a continuation of the Spinal 

 Cord, modified in its function : so that, when we trace upwards the cavity of 

 the spinal column into that of the cranium, we encounter no material change, 

 either in its size or 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 intervertebral, are evidently more analogous to the Spinal nerves, both 

 in situation and function. A separation of the primitive fibres of these takes 

 place, however, 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 ante- 

 rior 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 of this nerve with those proceeding more directly 

 from the medulla oblongata, which we find in Fishes and in some Amphibia. 

 According to Valentin, the Glosso-pharyngeal is the sensory portion of the 

 first pair from the medulla oblongata, of which the motor part is chiefly com- 

 prehended in the Facial nerve. It is very interesting to trace this gradual 

 metamorphosis from the character of the Spinal nerves, which is exhibited in 

 the Cephalic, when they are traced upwards from the Medulla Oblongata; and 

 this is shown, as formerly pointed out ( 223), as much in the nerves of special 

 sensation as in the rest. Although we are accustomed to consider the Fifth 

 pair as par excellence the Spinal nerve of the head, the foregoing statements, 

 founded upon the history of its development, show that the nerves of the Orbit 

 really belong to its motor portion ; they may consequently be regarded as alto- 

 gether forming the first of the intervertebral or Spinal nerves of the cranium. 

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

 Par Vagum and Spinal Accessory intervene between this and the true Spinal, 

 of which the Hypoglossal may be considered as the first. 

 / 



XV. Motor Nerves of the Orbit. 



244. We now return to consider the functions of the Third, Fourth and 

 Sixth pairs of nerves, together constituting the entire channel of the movements 

 of the eyeball. Their particular functions are but ill understood, and the 

 movements which they govern offer so many peculiarities, that the inquiry 

 becomes a very complex one. It is of peculiar interest, however, both on 

 account of its general bearing on the Physiology of the Nervous System, and 

 at the present time, more especially, in consequence of the assistance iwhich 

 a correct Jmowledge of these functions may afford, in the treatment of Stra- 



