THE KERVOUS SYSTEM. 



589 



becomes larger both laterally and antero-posteriorly, and after a time 

 opens out on the dorsal surface into a space which is known as the fourth 

 ventricle, and from being a cylinder with a central canal, it is flattened 

 out on one surface by the gradual approach of the central canal to that 



Fig. 360. Dorsal or posterior view of the medulla, fourth ventricle, and mesencephalon 

 (natural size), p.n. , line of the posterior roots of the spinal nerves; p.m.f., posterior median 

 fissure ; /. g. , funiculus gracilis; cl. , its clava; f.c. , funiculus cuneatus; f.R. , funiculus of 

 Rolando; r. b. , restiform body; c. s. , calamus scriptorius: Z, section of ligula or taenia; part of 

 choroid plexus is seen beneath it; l.r. , lateral recess of the ventricle; sir. , striae acusticae; i.f. , 

 inferior fossa; s.f. , posterior fossa; between it and the median sulcus is the fasciculus teres; 

 cbl. , cut surface of the cerebellar hemisphere; nd. , central or gray matter; s. m.v., superior 

 medullary velum ; Ing. , ligula; s.c.p., superior cerebellar peduncle cut longitudinally; cr., 

 combined section of the three cerebellar peduncles ; c. q. s. , c. q. i. , corpora quadrigemina (su- 

 perior 

 turn; ( 

 thalamus 

 (E. A. SchSfer?) 



surface. The central canal of the cord, therefore, is directly continuous 

 with the fourth ventricle. 



If the bulb be examined on its anterior or ventral surface it is found 

 that the anterior fissure, which is a continuation of the same fissure in 

 the cord, is occupied at the most posterior part by fibres which are 

 crossing from one side to the other ; the central canal being pushed now 

 toward the posterior surface. This is what is known as the anterior 

 decussation of the medulla oblongata. It is formed of the fibres which 

 in the cord occupy the postero-lateral region and are called the crossed 

 pyramidal fibres. The lateral pyramidal fibres of either side after cross- 

 ing in the middle line in this way become part of the anterior pyramid 



