626 FUNCTIONS OF THE CEREBRO - SPIN AL NERVOUS SYSTEM. 



the study of Embryonic development, alike show that the Spinal Cord and 

 Medulla Oblongata constitute the most essential part of the nervous system 

 in Vertebrata ; and that the Cerebral Hemispheres arc superadded, as it were, 

 to this. At an early period of development, the Eucephalon consists chiefly 

 of four vesicles, which correspond with the ganglionic enlargements of the 

 nervous cord of the Articulata, and mark four divisions of the Cerebro-Spiual 

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

 bones of the cranium, elements which present an analogy to those that would 

 form four vertebne, in a much expanded and altered condition. The four 

 pairs of nerves of special sensation, Auditory, Gustatory, Optic, and Olfac- 

 tory, make their way out through these four cranial vertebrse 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 dis- 

 tribution appears irregular. Thus the greater part of the sensory fibres are 

 contained in the large division of the Trigemiuus; whilst of the motor fibres, 

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

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

 with the first pair from the Medulla Oblougata 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 

 Oblongata. 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 comprehended in the Facial nerve. Although we are ac- 

 customed to consider the Fifth pair as par eminence the Spinal nerve of the 

 head, the foregoing statements, founded upon the history of development, 1 

 show that the nerves of the Orbit really belong to its motor portion ; they may 

 consequently be regarded as altogether forming the first of the intervertebral 

 nerves of the cranium. The Facial and Glosso-pharyugeal appear to con- 

 stitute the second ; whilst the Par Vagum and Spinal Accessory, forming the 

 third pair, intervene between this and the two Spinal, of which the Hypo- 

 glossal may be considered as the first. 



499. Functions of the Spinal Axis. In considering the functions of the 

 Spinal Cord, we have to regard it under two aspects ; in the first place, as 

 a conductor of nervous force between the Nerve-trunks and the Encephalic 

 centres ; and in the second place, as itself an, independent centre of nervous 

 power. As a mere conductor of nervous force, its functions are the same as 

 those of a nerve-trunk : 2 for if it be divided, all the parts of the body which 

 are solely supplied by nerves coming oft' below the point of section are 

 completely paralyzed, as far as regards sensibility and voluntary movement; 

 no impressions made upon them having the least power to affect the con- 

 sciousness, and no exertion of the will being able to determine contraction 

 of their muscles. This state of paraplegia, which may be experimentally 

 induced in animals, is frequently exhibited in Man as a result of injury or 

 "i" disease which seriously implicates the Spinal Cord; and as it has been 

 shown that among the lower animals complete reunion of the Cord may take 

 place after complete division, as indicated by the entire restoration of its 

 functional powers and the complete redintegration of its structure," so have 



1 On this point, as well as on the function^ of the Cephalic nerves generally, see Prof. 

 Valentin, l)i- Fnnct.ionihiis Nervornm Cerebralium et Nervi Sympathies, Berna-, 1839. 



1 Valentin (Die Zuckungsgesetzu des lebenden Nerven mid Bluskels, IHii:!, p. 86) 

 observes that, the whole spinal cord behaves like a lar^e nerve on applying ekctrical 

 stimulation to it, though its irritability H wry rapidly exhausted. 



3 See the admirable researches of M. Brown-Sdquard, in Gazette Medicale, 18J9, 



