THE SPINAL CORD. 247 



matter, open towards the centre of the medulla, and named the cor- 

 pus dentatum of the olivary body; but there are numerous white fibres, 

 within, and "around, this ganglionic centre ; the external fibres are 

 continued downwards into the antero-lateral columns of the cord, and 

 upwards into the under part of the cerebral peduncle; the fibres 

 which proceed from the interior of the corpus dentatum, ascend to the 

 corpora quadrigemina. The lateral columns, also composed of white 

 fibres, descend from the cerebral, and middle cerebellar peduncles, to 

 the sides of the cord, partly undergoing decussation below, and form- 

 ing a transverse commissure above, behind the corpora quadrigemina. 

 The restiform bodies consist of white fibres, including a ganglionic 

 mass of gray matter; they connect the inferior peduncles of the cere- 

 bellum with the posterior and lateral columns of the cord. The pos- 

 terior pyramids, composed of white fibres, descend from the upper or 

 back part of the cerebral peduncles, to the posterior part of the lateral, 

 and to the posterior columns of the cord ; some of these fibres are said 

 to decussate opposite the back of the pons. Embracing the upper part 

 of the two halves of the medulla oblongata, are certain transverse sets 

 of fibres, superficial and deep, named arciform fibres, which serve to 

 connect together, not only the two halves of the medulla, but all its 

 component masses of gray matter; many of these fibres are associated 

 especially with the corpora dentata of the olivary bodies. 



The gray matter of the pons, as already stated, is diffused amongst 

 the longitudinal and transverse white fibres ; but in the medulla ob- 

 longata, it is collected together into more compact and definite masses. 

 Of these, the folded corpus dentatum of the olivary body, has already 

 been described; the tubercle of Rolando incloses a rounded mass, 

 which is continuous below, with the so-called posterior cornu of the 

 gray matter of the spinal cord; the rest of the gray matter of the 

 medulla is chiefly collected in symmetrical masses, situated in its 

 posterior portion, closely contiguous, and more or less blended to- 

 gether; they constitute special ganglionic centres of origin of most 

 important cranial nerves. In the lower part of the medulla oblongata, 

 the gray matter becomes more concentrated, and more covered in be- 

 hind by the white substance, until at length, it passes into the completely 

 inclosed gray matter of the cord. 



From the preceding account, it will be seen that the medulla oblon- 

 gata, like the cerebral and cerebellar peduncles and the pons, is white 

 externally; but it has gray matter intermixed with all its component 

 parts, except with the white fibres of the anterior pyramid; moreover, 

 the gray matter approaches very closely the posterior surface of the 

 medulla, where it seems, as it were, to have its interior opened out. 



The spinal cord. The spinal cord, Fig. 12, c, Fig. 60, c, c, a cylin- 

 drical mass of nerve substance, forms the prolongation downwards of 

 the medulla oblongata. It presents a shallow, open, anterior median 

 fissure, and a deep, close, posterior median fissure, which mark it off 

 into a right and left half, united together by a narrow, deep, central 

 commissural part. In each half, are two slight longitudinal lines, 

 serving to distinguish it into what are called the anterior, lateral, and 

 posterior columns, a narrow band of the latter, next to the posterior 



