DISSECTION OF THE DOG 223 



under the pons. Research has shown that the fibres composing the 

 pyramid are derived from nerve cells in the grey matter of that part of the 

 cerebral cortex which lies in the neighbourhood of the cruciate sulcus. From 

 their origin these fibres, which are motor in function, pass by way of the corona 

 radiata and internal capsule into the basis of the cerebral peduncle and so 

 through the pons into the pyramid of the medulla oblongata. Close to the 

 foramen magnum the fibres cross over from one side to the other (decussation 

 of the pyramids) and travel along the crossed 'pyramidal tract of the opposite 

 side of the cord. 



Lateral to the pyramid is a groove, of no great depth, from which the rootlets 

 of the abducent and hypoglossal nerves arise. The groove separates the 

 pyramid from an elevation known as the facial tubercle (tuberculum faciale). 

 In an adult dog the tubercle is not very conspicuous, because its surface is 

 crossed by the external arcuate fibres (fibrae arcuatse externse) which traverse 

 the lateral border of the medulla oblongata obliquely. Behind the facial 

 tubercle the medulla oblongata resembles the lateral part of the spinal cord, 

 and to the naked eye it appears as if the lateral columns of the cord were 

 merely prolonged into the medulla. This appearance is, however, contrary 

 to fact. The lateral column of the cord contains fibres not present in a similar 

 position in the medulla, e.g. the crossed pyramidal tract. 



On each side of the dorsal median fissure is a very narrow strand of fibres 

 forming the funiculus gracilis, which deviates from the middle line when the 

 end of the fourth ventricle is reached and expands slightly to form the clava. 

 Lateral to the gracile funicle is the much wider funiculus cuneatus. This 

 also diverges laterally on reaching the ventricle and forms the eminence 

 called the tuberculum cinereum. The gracile and cuneate funicles form 

 the direct continuation of the dorsal column of the spinal cord. The clava 

 and tuberculum cinereum contain collections of nerve cells (nucleus funiculi 

 gracilis et nucleus funiculi cuneati) in association with which the fibres of the 

 dorsal column of the cord end. 



A third band of white matter lies lateral to the funiculus cuneatus. This 

 is known as the funiculus of Rolando because it is produced by the substantia 

 gelationosa [Rolandi] of the cord coming towards the surface when the medulla 

 oblongata is reached. 



A superficial naked-eye examination of the medulla oblongata leads to 

 the conclusion — an erroneous one — that the three funicles just mentioned are 

 continued forwards as the restiform body (corpus restif orme) , a thick strand 

 which, after forming a floor for the lateral prolongation of the fourth ventricle, 

 turns abruptly dorsalwards and enters the cerebellum. It will be observed 

 that the external arcuate fibres enter largely into the formation of the restiform 

 body. 



Pons. — The pons is a transverse prominence between the medulla oblongata 

 and the cerebral peduncles. Its ventral surface is convex from side to side 



