504 OF THE NERVOUS SYSTEM AN*D ITS ACTIONS. 



considered as making up, with the preceding, an ordinary spinal nerve. 

 The Spinal Accessory, again, appears to be chiefly or entirely a motor 

 nerve at its origin; and in like manner the Pneumogastric, or Par 

 Vagum, seems at its roots to correspond with the posterior roots of the 

 ordinary spinal nerves, and to execute functions analogous to theirs ; 

 but these two nerves exchange fibres, so that each acquires in part the 

 endowments of the other. The Facial nerve (or portio dura of the 7th), 

 which is the nerve that supplies the muscles of the head in general, 

 arises by a single root, and is exclusively motor in its properties, 

 except in branches which have received sensory filaments by inoscula- 

 tion with other nerves. The same is the case, also, with the Motor 

 .Nerves of the Orbit (the 6th, 4th, and 3d, of Willis), which arise by 

 single roots, and which have no sensory endowments but those which 

 they obtain by inosculation with the Fifth pair. On the other hand, 

 the Fifth pair arises by a double root ; that which corresponds to the 

 anterior or motor root of the spinal nerves is very small, however, and 

 only enters the third division of the nerve, which supplies the muscles 

 concerned in mastication ; the other root, corresponding with the pos- 

 terior roots of the spinal nerves, is of large size, and its branches are 

 distributed to the face and head, endowing them with sensibility. Thus 

 the sensory division of the fifth pair, being distributed, not merely to 

 the same parts with its motor division, but also to the parts which de- 

 rive their motor endowments from the Facial nerve, and from the 

 nerves of the orbit, may be regarded as making up, together with all of 

 them, one ordinary Spinal nerve. 



4. Functions of the Medulla- Oblong ata. 



889. This portion of the nervous centres, as already stated, does not 

 differ in any essential particular from the Spinal Cord, of which it may 

 be considered as a cranial prolongation. But the arrangement of its 

 constituent parts is peculiar ; for whilst it is the medium by which the 

 various strands of the Spinal Cord are connected, with the different 

 portions of the Encephalon, it is also remarkable as being the gan- 

 glionic centre, concerned in the maintenance of the action of respi- 

 ration, and in the ingestion of food. Four principal strands of nervous 

 matter may be distinguished anatomically, in each of its lateral halves ; 

 these are, anteriorly, the Anterior Pyramids; next, the Olivary 

 bodies ; next the Restiform bodies ; and lastly, the Posterior Pyramids. 

 It will be presently seen, however, that the physiological relations of 

 these strands, as indicated by their connexions with the Encephalon 

 above, with the Spinal Cord below, and with the nerves that have their 

 centres in them, are very different from what their mere relative posi- 

 tions would indicate. The gray or vesicular substance in this part no 

 longer holds the same relation to the white, that it possesses in the 

 Spinal Cord ; but is principally aggregated in three pairs of gangli- 

 onic centres, of which the anterior forms the nucleus of the Olivary 

 body, the lateral of the Restiform, and the posterior of the Posterior 

 Pyramidal. 



890. The Anterior Pyramids consist entirely of fibrous structure, 



