812 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



oblongata and appear to the naked eye as the immediate continuation of 

 the posterior white columns (postero-external fibres) of the spinal cord. 

 At the level of the foramen magnum they divide and expose the interior 

 gray substance of the spinal cord, which, previously covered by the pos- 

 terior columns, here becomes exposed on the posterior surface. The gray 

 matter at this point also becomes flattened out through the opening of the 

 central canal of the spinal cord, its borders being flattened out laterally. 

 The posterior pyramids form the upward continuation of the postero- 

 median fibres of the posterior columns of the cord. 



While the gray substance of the spinal cord forms a compact mass 

 traversed only by the narrow central canal and is everywhere surrounded 

 by a sheath of white substance, in the medulla it becomes exposed and 

 forms a flattened layer, and is divided into two symmetrical halves lying 

 on each side of the median line and bounded only by a slight rim, the 

 last trace of the central spinal canal. This exposed surface of gray 

 matter in the medulla oblongata has the form of an elongated rhomb, 

 whose longest diagonal coincides with the median line, and it forms the 

 floor of the fourth ventricle, communicating above with the aqueduct of 

 Sylvius and below with the central canal of the spinal cord, the lower 

 pointed end of this surface bearing the name of the calamus scriptorius. 

 The gray substance of the floor of the fourth ventricle is entirely similar 

 to that of the spinal cord ; it contains a large mass of multipolar gan- 

 glion cells, of which a few form distinct roots and appear to be in direct 

 communication with the cranial motor nerves. 



The ganglionic origin of the sensory cranial nerves is less distinctly 

 made out. It is, however, clear that all the cranial nerves, from the 

 oculo-motor to the hypoglossal, originate in the medulla oblongata and 

 its annexes. Only the optic and olfactory nerves originate within the 

 cerebrum. The gray matter of the floor of the fourth ventricle forms a 

 direct continuation of the gray matter of the spinal cord, and is to this 

 extent analogous to the anterior columns, the p3^ramids, and cerebellar 

 fibres, which pass without interruption from the spinal cord into the 

 medulla oblongata. 



The anterior divisions of the lateral columns of the spinal cord 

 likewise pass without interruption into the medulla oblongata and force 

 themselves between the olivary body and restiform bod}'', without, how- 

 ever, passing farther toward the median line (Fig. 352). 



From the raphe originate numerous nervous fibres, or so-called 

 internal transverse fibres, which cross the longitudinal fibres of the 

 medulla at a right angle and split up into a large number of smaller 

 bundles, the net-work so formed being described as the reticular forma- 

 tion of the medulla oblongata. This net-work incloses in its spaces 

 numerous multipolar ganglion cells, in which terminate, in all probabilit}-, 



