THE BRAIN AND SPINAL COED. 215 



side than on the right, probably owing to the centres for speech 

 being situated on the left side. The temporal operculum rises first 

 (5th month) the others a month later. The opercula which bound 

 the posterior horizontal limb of the fissure of Sylvius are the first 



anter. limb 



post, horiz. limb. 

 JF/ 'fronto-par. open. 



■ stem of fissure 

 vallecula Syluii 



p. triang. ant. asc. limb „--•' ^ 



ant. horiz. i ~~V\/^ ~$. <^<^/ 



limb. \ \\ <^^-^- mthQrizJimb 



pars orbit. 



i. 



vallecula Syluii 



Fiq. 177. — Diagram of the Opercula and Fissure of Sylvius. 

 In A the orbital operculum is undivided ; in B it is subdivided. (After Cunningham.) 



to come in contact. By the end of the first year after birth all 

 three opercula meet over the Island and completely hide it. At 

 birth there is still a part of the Island exposed behind the 

 orbital operculum and in lower human races this is frequently 

 the condition throughout life. The orbital opercula (pars 

 triangularis and pars orbitalis) become part of the centre of 

 speech and represent the latest additions to the human brain. 

 If the orbital operculum is not subdivided, which is commonly 

 the condition on the right hemisphere, then the anterior limb of 

 the fissure of Sylvius is not subdivided into anterior horizontal 

 and ascending parts (Fig. 177 ^4 and B). 



Comparative Anatomy of the Opercula and Island. — The 



