130 THE HUMAN BODY. 



one-half of the spinal marrow to the other, and of which we 

 shall speak presently. The posterior furrow, like the anterior, 

 disappears insensibly toward the inferior extremity of the 

 spinal cord; at its upper extremity it opens at an acute angle 

 where the medulla oblongata commences. Its form re- 

 sembles the point of a pen hence the name calamus scrip- 

 torius which has been given to this part of the medulla 

 oblongata. 



Each half of the spinal cord, separated from the other by 

 the fissures indicated above, is composed of two cords or 

 bundles; one, the posterior, giving origin to the posterior 

 roots of the nerves, and the other, the anterior, to the 

 anterior roots. These cords are attached to, and are con- 

 tinuations of, the pyramids of the medulla oblongata. 



This latter is marked in front by the median furrow which 

 extends beyond the interlacing of the fibres, of which men- 

 tion has been made; on each side of this furrow there is 

 an oblong elevation, these are the anterior pyramids ; outside 

 of which are two projections still more marked, called olives 

 or the olivary bodies. Laterally there is a depression, grayish 

 in colour, in which terminate the posterior roots of the spinal 

 nerves; behind this we find a bundle of distinct fibres, the 

 restiform or cord-shaped bodies ; and lastly, outside of these 

 bodies are fat posterior pyramids > defining the calamus scrip- 

 torius on either side. The cerebellum, as we have already 

 seen, covers the posterior surface of the bulb, to which it is 

 united by the restiform bodies or inferior peduncles of the 

 cerebellum, and which contribute with the cavity of the 

 calamus scriptorius to form the fourth ventricle. 



The anterior pyramids terminate by the interlacing of their 

 nervous fascicles, and this interlacement may be considered 

 as the lower boundary of the medulla oblongata. These 

 pyramids are contracted at the apex and at the base, and are 

 inserted into the pons Varolii by a sort of neck or contraction. 



The anterior and posterior roots of the spinal nerves form 

 two parallel lines on the sides of the spinal marrow. The 

 roots spring from the spinal cord, but it cannot be ana- 

 tomically demonstrated that their fibres return to it beyond 

 the point where they originate and where they constitute by 



