100 THE VOYAGE OF H.M.S. CHALLENGER. 



thalami. In Phoca vitulina the pineal body also projected behind the corpus callosum, 

 and resembled in shape and in its relations to corpora quadrigemina, cerebrum, and cere- 

 bellum the epiphysis cerebri in the Elephant Seal. It was 1G mm. long, 8 mm. in greatest 

 breadth, and 6 mm. in greatest vertical diameter. 1 The length of the cerebrum in this 

 specimen was 78 mm. (3 inches). Dr. James Murie is, I think, the only anatomist who 

 has systematically described the brain of an Eared Seal, and he states that in Otaria 

 jubata the jjineal body is " relatively large," but he does not give its actual dimensions, 

 though, if I may judge of its size as represented in his fig. 44, it does not seem 

 to have been more than about 8 mm. long. It would appear, therefore, that in the 

 Seals this body is undoubtedly larger than in Mammals generally, though, as will be 

 shown later on, it is when largest in them only about one-half as big as in the Walrus, 

 and does not project so far back as to be visible between the two hemispheres of the 

 cerebrum. 



Cerebellum. — This was of large size, and consisted of a middle and of two lateral lobes. 

 On the tentorial aspect of the cerebellum the middle lobe was greatly elevated above the 

 lateral lobes, and from its summit the surface sloped rapidly downwards and outwards to 

 the sides of the organ. At the superior border of the cerebellum, which corresponded to 

 the ossified tentorium, there was a slight notch opposite the termination of the middle 

 lobe. On the ventral surface the middle lobe formed the roof of the 4th ventricle and 

 was situated in a fossa between the two lateral lobes. The middle lobe was separated in 

 the greater part of its extent from the lateral lobes or hemispheres by a deep fissure on 

 each side. Each lateral lobe, much thicker when in apposition with the middle lobe 

 than at the borders of the hemisphere, was separated into a tentorial and an occipital 

 surface by a deep fissure, which corresponded to the great horizontal fissure of the human 

 cerebellum, but owing to the different plane occupied by the cerebellum in the Elephant 

 Seal, it may more appropriately be called the vertical transverse Jissure. The sur- 

 face of both the middle and lateral lobes was subdivided into numerous folia, but as 

 this surface was much broken up by fissures possessing considerable depth, and often 

 tortuous in direction, the folia were short, and did not have the broad plate-like character 

 one sees in the human cerebellum. These fissures were especially marked on the occipital 

 surface of the hemispheres, on which they ran from within outwards, but were not quite 

 symmetrical on the two sides. 



Pons Varolii. — The pons had the usual form. Its mesial line on the ventral surface 

 was marked by a shallow groove for the basilar artery, and this surface consisted of the 

 superficial transverse fibres. It gave origin at the posterior part of its lateral and ventral 

 aspect to the two roots of the 5th nerve, the motor root being immediately internal to the 

 sensory. The sensory root was much thicker than the motor, and its fasciculi were 



1 That the pineal gland in Phoca is larger than is usual in the Mammalia was recognised by Ehlers, Zeitschr.f. wiss. 

 Zool, Bd. xxx. p. 628, Supplement, 1878. 



