590 ORIGIN OF THE ENCEPHALIC NERVES. 



On the Immediate Origin of the Encephalic Nerves. 



— By reference to fig. 225, it will be seen that all the ence- 

 phalic nerves, (with the exception perhaps of the olfactory,) 

 originate from portions of the base of the brain, into which 

 has been directly traced the fibres of the spinal cord. It has 

 been shown that the nerves arising from the spinal medulla, 

 consist of two distinct classes, those of sensation and those of 

 motion : and that each nerve is, in all probability, composed in 

 part of filaments which, reflected at their origin from the gray 

 substance of the spinal cord, have no connexion with the 

 brain, and are called the reflex, or excito-motor ; and -partly of 

 those which are directly continuous with the brain, called the 

 sensiferous or cerebral. The encephalic nerves may be justly 

 considered as originating much in the same manner, with the 

 exception that their sensitive and motor roots, are, in general, 

 continued on as distinct trunks, to their termination in the organs. 

 — Their origin is derived in a great part from the medulla 

 oblongata, which contains a large amount of cineritious matter, 

 and from whence there is reflected a greater amount of nervous 

 fibrils, than from any similar bulk of the spinal cord ; some of 

 their fibres being also continued upwards to the brain, like the 

 sensiferous of the spinal cord. If it were possible to isolate dis- 

 tinctly the sensory from the motory fibres of the nerves, it 

 would be better to describe them in the direction in which they 

 act, the sensory from without inwards, and the motory from 

 the central organs towards the circumference. As this cannot 

 be done, they are considered from convenience of description, 

 as all originating from the central organs. 



— The hypoglossal arises in the sulcus between the corpus 

 pyramidale and corpus olivare, by three roots placed in a line 

 with the anterior roots of the spinal nerves, which emerge by 

 a number of minute filaments from the anterior columns of the 

 medulla oblongata. It is a motor nerve. 



— The spinal accessory of Willis, as has been observed, (page 

 464,) arises from the cervical portion of the spinal marrow. 

 — It commences from the lateral part of the cord, usually oppo- 

 site the fifth cervical vertebra, behind the ligamentum den- 

 ticulatum, and near the posterior roots of the spinal nerves. 



