

FUNCTIONS OP THE SPINAL AXIS. 695 



sation Auditory, Gustatory, Optic, and Olfactory 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 separa- 

 tion 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 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 Oblongata. According to Valentin, the Grlosso- 

 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 accustomed to consider the Fifth pair as par eminence the Spinal nerve, 

 of the head, the foregoing statements, founded upon the history of develop- 

 ment, 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 inter- 

 vertebral nerves of the cranium. The Facial and Grlosso-pharyngeal appear to 

 constitute the second ; whilst the Par Vaguin and Spinal Accessory, forming 

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

 glossal may be considered as the first. 



723. Functions of the Spinal Axis. Whatever view we may take of the 

 structure of the Spinal Cord, no doubt can be fairly entertained that it must be 

 physiologically treated on the one hand as a true centre (or rather as an aggre- 

 gation of separate centres) of nervous power, and on the other as a medium of 

 conduction between the Encephalic centres and the roots of the Spinal nerves. 

 And although its attributes as an independent centre become most obvious when 

 it is separated from the rest, yet there can be no reasonable doubt that it is 

 always acting as such, even when every part of the Nervous System is in a state 

 of complete vigor. It may, in fact, be said to supply, by its " reflex power," 

 the conditions requisite for the maintenance of the various Organic processes ; 

 and, as Dr. M. Hall has pointed out, it especially governs the various orifices of 

 ingress and egress. Thus the act of Deglutition is entirely dependent upon 

 the Spinal Axis and the nerves proceeding from it ; the Will being in no other 

 way concerned in it than by originating the necessary stimulus ; and even sen- 

 sation not being a necessary link in the chain of excito-motor action ( 426 

 428). The action of the cardiac sphincter, again and probably that of the 

 pyloric sphincter also is dependent upon its nervous connection with the 

 Spinal Axis ; and is entirely regulated without sensorial excitement ( 428). 

 And there is much reason to believe that certain of the movements of the 

 Stomach itself are in like manner dependent upon its connection with the Me- 

 dulla Oblongata ( 480) ] although it unquestionably possesses an independent 

 motor activity of its own. The movements of the Intestinal tube are undoubt- 

 edly influenced by the Spinal Cord, although essentially independent of it 

 ( 432, 433) ; but the sphincter which surrounds its orifice of egress is un- 

 doubtedly placed under its guardianship, although partly subjected (in Man) to 

 the control of the Will. The same may be said of the expulsor muscles con- 

 cerned in the act of Defecation ; and of the expulsors and sphincter which 

 effect and control the act of Urination ( 434). Looking, again, at the move- 

 ments which are subservient to the Respiratory process, we find that all those 



1 See Prof. Valentin "De Functionibus Nervorum Cerebralium et Nervi Sympathetic!," 

 Berrise, 1839 ; lib. iii. cap. i. 



