NERVOUS SYSTEM. VERTEBHATA. 



L88 



It is the sulcus frontalis medius. and is seen in its primitive 

 and typical form on the left hemisphere of Ilylobates 

 leucogenys (D. G46), and also on the left hemisphere of 

 this Orang just above the triradriate sulcus, which, with :i 

 reservation, has been called " rectus." Yet on the ri^ht 

 hemisphere the function of this sulcus frontalis medius ha- 

 obviously been taken partly by the sulcus frontalis inferior 

 (rectus) and partly by the anterior end of the superior 

 frontal. The extensive, deep, superior precentral and 

 superior frontal sulci are united to form a triradiate pattern, 

 and are arranged with quite diagrammatic simplicity. 



Between the superior frontal sulcus and the mesial edge 

 there are two small shallow sulci, which can only reprex nt 

 a "sulcus frontalis mesialis," which, so Cunningham 

 believes, " never occurs in the cerebral hemisphere of the 

 Apes " (Mem. Roy. Irish Acad. 1892, p. 296). 



The parieto-occipital sulcus is very deeply and irregu- 

 larly incised, and if its lips be separated (or even without 

 doing so in the right hemisphere) the dual composition of 

 the sulcus (which was explained in the case of Papio) 

 is rendered evident. The sulcus has a much greater 

 dorsal extent (see especially the left hemisphere) than in 

 any of the Cercopithecidse. The elongated arcus occipitalis 

 is wholly exposed on the left hemisphere, so that it has the 

 appearance of having pushed back the occipital operculutn. 

 On the right side, however, the posterior limb of the arcus 

 is hidden under the operculum. 



In the right hemisphere there is an ideally complete 

 intraparietal sulcus, with a very long ramus postcentrali.- 

 superior and ramus occipitalis transversus, which is con- 

 current with the Simian sulcus. There are in addition 

 compensatory sulci parallel to and on both sides of the 

 ramus horizontalis and in front of the ramus postcentralis 

 inferior. 



On the left hemisphere, the intraparietal sulcus presents 

 a condition such as often occurs in Man, but never (so far 

 as I am aware) in lower Apes. The ramus postcentralis 

 superior, and not the inferior, is joined to the ramus 

 horizontalis. 



The upper end of the parallel sulcus forms a hook-like 



2 F 



VOL. II. 



