082 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvi. 



right liemicerebrum the marginal g3'^re should be included. The pre- 

 svlvian has a greater area than the postsylvian region. The former is 

 made up mostly of the medilateral and siipersylvian gyres and the 

 latter by the sylvian and sui)ersylvian gyres. 



MESAL ASPECT. 



I/ij)j>()c<uiq><il ^fixxni'c. — This occupies the usual position. It is seeji 

 arching from the splenium of the callosum around the optic thalamus 

 to the ti]) of the pyriform or temporal lo))e. 



CdlloKdl ijss}(r('. — As its name indicates, it is closely related to the 

 callosum. It separates the callosum from the adjacent cortex. It is 

 deepest in the region of the splenium and gradually grows shallower 

 until as it curves around the genu it becomes flush with the surface. 

 On the right hemicerebrum it diverges somewhat from the genu of 

 the callosum. 



Sjdeii/al ^-/issure. — This fissure is wxll developed in the walrus and 

 occupies the usual position upon the tentorial surface of the l)rain. It 

 arches upward and forward, curving around the splenium on the mesal 

 surface. On the right hemicerebrum it is a continuous fissure and 

 extends as far as the frontal portion of the callosum; it then extends 

 upw^ard almost vertically nearly to the dorsal margin of the hemicere- 

 brum. On the left hemicerebrum the fissure is interrupted. Its ten- 

 torial portion stops at the level of the splenium. The mesal portion 

 begins as two small superficial fissures converging in a fork-like man- 

 ner to form the mesal splenial proper. At the level of the genu it 

 terminates in a fork, the lower branch passing nearl}^ to the frontal 

 margin of the hemicerebrum. In neither case did the splenial con- 

 nect with the cruciate, as described by Turner in his specimens. On 

 the right hemicerebrum there was a slight indication of a superficial 

 conn(K-ti()n. but a subi:5ierged gyre or l)uttress shut off any free 

 connuunication. j 



Wppocmnpal gyre. — .This gyre lies upon the tentorial surface of the 

 brain. It forms the mos-^v of the mesal portion of the pyriform or hip- 

 pocampal lobe. It take,- the same general direction as the hippocampal 

 fissure (which forms its jphalic or inner boundary), arching upward 

 to the lev(d of the splei Am. Its caudal boundary is formed by the 

 tentoi-ial portion of the-^pJ6{iial fissure. The hippocampal gyre aver- 

 ages 15 millimeters in width and possesses a few minor fissures, which 

 in the main are ofl'shoots froi the splenial and have a horizontal direc- 

 tion. On the left hemicorel^r.um, 1} ing in the hippocampal gyre just 

 caudal to the splenium, is a \j,yvy shallow sulcus corresponding in posi- 

 tion to the fissura subhmica^ ti Kiikenthal. It is not noticeable upon 

 the right hemicerebrum. ■,>! 



Callosal gyre. — This, as the ^jiame indicates, lies just dorsal to the 

 callosum. It is a narrow gyre and its average width is about 10 milli- 



