the anatomy of a hammerhead shark, 979 



Brain and Cranial Nerves. 



Busch (2) and Miklucho-Maclay (12) have briefly, and on the 

 whole accurately, described and figured the macroscopic structure 

 of the brain. Owing to the poor condition of our specimen we 

 are not able to add much to their account, but there are a few 

 points on which our observations do not agree with theirs. 



The brain is antero-posteriorly shortened and, in dorsal view, 

 only a portion of the olfactory lobes (o.l.), the p7~osencephcdon ( f.b.), 

 the much convoluted cerebellum {c), the restiform bodies (r.b.), and 

 the short medulla oblongata (m.) are visible (text-fig. 7). The 

 diencephalon and optic lobes are completely covered by the 

 cerebellum. 



The prosencephalon is single and shows no trace of a median 

 fissure. 



The large paired olfactory lobes are situated ventral to the 

 prosencephalon ; only their anterior ends being visible in dorsal 

 view. 



Owing to the intimate relation of the olfactory lobes and the 

 prosencephalon Busch (3) has named them the " lobes communes," 

 and a section through the forebrain appears to jvistify this name. 

 On the other hand, in his diagram he shows the prosencephalon 

 as a paired structure and the olfactory lobes as unpaired. This 

 is opposed to our observations. 



The cerebellum is roughly triangular in shape, the base of the 

 triangle being closely apposed to the posterior of the prosen- 

 cephalon, and its apex posterior. It is primarily divided into 

 three segments ; two of these being anterior, right and left, and 

 the third running obliquely backward from the right. The left 

 anterior segment is approximately equal in size to the posterior 

 segment, but the right anterior segment is considerably smaller. 

 All three segments are further divided into smaller segments, 

 which in turn are sulcated. 



Ventral to the posterior end of the cerebellum are two small 

 oval lobes called by Miklucho-Maclay (12) the lobi tingemini (It.). 



The restiform bodies (r.b.) are well developed and occupj^ their 

 normal position. The Jiiedulla oblongata (m.) is very short, and 

 the fourth ventricle is correspondingly reduced. 



In venti-al view the olfactory lobes completely cover the 

 prosencephalon. The optic chiasma lies immediately posterior to 

 the olfactory lobes. 



The infundibulum is well developed, and the saccus vasculosus 

 shows on each side. Portions of the optic lobes are also visible in 

 this view. 



An examination of a median longitudinal section confirms 

 Miklucho-Maclay's observation that a wedge-shaped portion of the 

 cerebellum projects into the ventr"icle of the " Zwischenhirn." 



The following Notes on the cranial nerves were made chiefly 

 during the preparation of the skull, and are consequently far from 

 complete. 



The olfactory nerve is large and is given off laterally from the 



