528 DR. J. MUKIE ON THE ANATOMY OF THE SEA-LION. 



ing cornu. The taenia semicircularis is hidden by the choroid plexus ; but when the 

 latter is raised it appears to be well-developed though Hat. That portion of the 

 thalamus opticus which is exposed in the lateral ventricle has a very elongated and 

 acutely pointed diamond-shape, its greatest length being IT inch, and the extreme 

 breadth 0-4 inch. Its outer posterior border is the highest part, from which the sur- 

 face gently shallows inwards and forwards. 



In the horizontally opened ventricle, the middle or descending cornu is hidden. But 

 on a vertical and transverse section being made behind the thalamus, through the 

 temporal lobe, or the same parts opened up as is shown in fig. 4G, the middle cornu is 

 observed to curve downwards perpendicularly, then forwards and inwards to the tip of 

 the temporal lobe. This remarkably vertical descending cornu has a depth or length 

 of T25 inch. The hippocampus major which forms the anterior wall is of considerable 

 size. Its surface, at the vertical upper portion, is flattened behind, and with a narrow 

 and compressed outer margin ; but as it extends inwards and forwards it becomes 

 altogether more equally rounded and convex. Superficially it is smooth, and devoid of 

 a pes hippocampi. The corpus fimbriatum and the continuation of the choroid plexus, 

 both of fair size, lie in the deep sulcus in front, and are in great part concealed by the 

 outstanding body of the hippocampus major. The posterior cornu stretches backwards 

 and outwards with a very regular sweeping arch, and goes well back into the occipital 

 lobe, terminating in a shallow tapering extremity. The eminentia collateralis is not 

 distinctly defined ; but what appears to represent the outwardly bulging hippocampus 

 minor has a length of 0'7 inch, and at widest is 0-3 to 0-4 broad. 



In the section under consideration, I measured the cerebrum, after the manner of 

 Mr. Flower, with a view to compare the dimensions of the anterior and posterior 

 regions. It yielded the following results: the front or anterior median region from 

 the point of junction of the hippocampi is equal to 2T inches, whilst the posterior 

 region from the same point is 1-55 inch long. This gives a proportion of 100 

 to 74. 



In the longitudinal, median, vertical section of the brain, the divided corpus callosum 

 is observed to occupy a nearly horizontal position slightly inclining downwards in front, 

 or with a very little tendency to a flattened arch. Relatively to the size of this median 

 face of the hemisphere it appears to be long and tolerably uniform in depth. The 

 rounded anterior genu possesses no special increment, as obtains in the Primates, but is 

 rather indented behind. The splenium or posterior fold, continuous with the fornix, is 

 likewise deficient in breadth, and turns abruptly at right angles downwards. The ex- 

 treme length of the corpus callosum in the preserved brain is 1*5 inch, its greatest 

 thickness - 2 inch, and its least thickness 0T inch. The anterior commissure is remark- 

 able on account of its diminutive size, having a circumference no greater than a pin's 

 head. The pineal gland, on the other hand, is relatively large ; the corpora quadri- 

 gemina intermediate as respects their volume. 



