MORPHOLOGY OF THE BRAI\ JX THE :\rAMMALTA. Ill 



Carnivores, the student next examines the brain in tlie Arctoidoa, he will find that the 

 Avhole of the first arcuate gyrus of Leuret has heconic submerg-ed in a great cleft in 

 the situation of the pseudosylvian sulcus, and that there is a pronounced tendency for the 

 anterior limb of the second arcuate gyrus to become submergtul. This tendency, which 

 is seen in most Bears, becomes more marked in Me/c\s; and in most of the Seals the 

 Avhole of the anterior limb of the second arcuate gyrus and the gi-(;ater poi-t of tin; 

 suprasylvian sulcus become swept into the pseudosylvian clcff. There is therefore in 

 the Arctoid Carnivores a tendency to the production of a coiulition which is analogous 

 to, witliout however being altogether identical with, the state of affairs which has 

 already become evolved in the Primates. It is of interest to note tliat when the supra- 

 sylvian sulcus is swept into the pseudosylvian sulcus in Dtarla, (hlob(r»iis [Trichechus], 

 and Fhoca, it becomes separated from the postsylvian sulcus (to which it is united in 

 most Carnivores). "When the suprasylvio-pseudosylvian comjjlex is formed in M'/nw- 

 cophafia, the postsylvian sulcus becomes disconnected ; but in the anomalous case {ckle 

 supra) in which the suprasylvian sulcus is not joined to the pseudosylvian, the former 

 sulcus was joined to the postsylvian. 



These peculiar phenomena are of great interest in the interpretation of the Prosimian 

 brain. It has frequently been noted in this memoir that in the Prosimiie, and especially 

 in the genus Lcmni; the mode of ending of the upper extremities of the suprasylviau 

 and pseudosylvian sulci is exceedingly variable, and indubitably points to a connection 

 between these two furrows, which has only recently been broken. In the peculiax-lv 

 interesting brain of Ch/roiin/.s, the suprasylvian and the variable pseudosylvian sulci are 

 widely separated, as in most quadrupeds; and, like the parallel case alforded by the 

 aberrant brain of Jff/nuecojj/nu/a, the suprasylvian sulcus tiien becomes joined (in most 

 cases) to the postsylvian sulcus. 



The extreme variability of the postsylvian sulcus in €'///' roii/i/s is chiefly the result of 

 the peculiar inconstancy of the pseudosylvian sulcus ; for it will be obvious, from an 

 examination of the brains of Procacia and tlie Ungulata, that the characters of the 

 postsylvian (which is a morphologically unstable sulcus) are determined chielly by the 

 nature of the pseudosylvian sulcus. The irregularity of the jjostsylvian sulcus mav 

 also be partly due to the fact that {Jhiroinys has obviously been subject to marked 

 retrogressive changes. 



In most Lemurs the postsylvian sulcus is a simple linear furrow, which obviousiy- 

 represents the parallel or superior temporal sulcus of the Apes and Man. 



Although the pseudosylvian sulcus has a morphological importance and stabilitv 

 which are immeasurably less than that of the suprasylvian sulcus, it is obvious that it 

 is the former which is the chief determinant of the peculiar " Sylvian formation." That 

 this is so is shown by a study of the Carnivora, in many of which (as, for instance, in 

 many Canidpe, the larger Pelidte, and Hysenidae) a condition so spuriously like that of 

 the true Sylvian fissure is produced without any participation by the suprasylvian sulcus, 

 that the mistake of regarding the Carnivore 2)seudosylvian sulcus and tiie Primate 

 Sylvian fissure as homologues is not unnatural. The suprasylvian sulcus, however; adds 

 stabilitv to a region which in other mammals is notoriouslv variable and iickie. Tlie 



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