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



from the basilar. The artery divided at the anterior border of the pons into three pairs 

 of branches, two of which passed to the tentorial surface of the cerebellum as the superior 

 cerebellar arteries, and the third pair passed to the tentorial surface of the cerebrum 

 behind the Sylvian fissure as the posterior cerebral arteries. At the inner end of the 

 Sylvian fossa was a short trunk, apparently the divided internal carotid artery, which was 

 joined with the posterior cerebral by a posterior communicating artery. From this short 

 trunk a middle cerebral or Sylvian artery ran outwards in the Sylvian fossa and fissure 

 to supply the cerebrum both in front of and behind the fissure ; another branch, an 

 anterior cerebral, ran forwards to the mesial longitudinal fissure, which it entered along 

 with its fellow of the opposite side, it ascended in front of the corpus callosum and then 

 ran backwards above it to supply the mesial face of the hemisphere. The two anterior 

 cerebrals were connected close to their origin by a large transverse anterior communi- 

 cating artery. A small branch, apparently a choroid artery for the supply of the choroid 

 plexus of the lateral ventricle, was seen at the inner end of the Sylvian fossa. In their 

 general arrangement these arteries at the base of the brain resembled the well-known 

 circle of Willis in the human brain. 



BRAIN OF WALRUS (Pis. VIII., IX., X.). 



Weight and External Form, of the Brain. — I have been fortunate to examine three 

 specimens of the brain of Trichechus rosmarus. The first was procured for me in 1865, 

 by my then pupil Mr. (now Dr.) Charles Moon of Dundee, from an animal (a) killed by 

 an officer of a whaling ship. I dissected it in the course of the following year, and 

 drawings were made in October 1866 by my then pupil Mr. (now Professor) Richard 

 Caton of Liverpool. The description of the brain and the drawings were at that time 

 reserved for future publication. Since then I have received two additional specimens, 1 

 one from a young animal (b), the other from a larger specimen (c). The following 

 description is based on an examination of all three specimens, and the drawings have been 

 revised with the help of the two additional brains. 



The brain of specimen a weighed after the removal of the membranes and hardening 

 in spirit 24 oz. 7 drachms avoirdupois ; that of b 13|- oz. ; that of c 26 oz. The two 

 cerebral hemispheres in c weighed 20^ oz.; the pons, medulla, and cerebellum 5f oz. 

 Brains a and c were therefore even after prolonged immersion in spirit heavier than the 

 brain of the specimen examined by Sir Richard Owen, which was probably weighed 

 immediately after removal. 



The principal dimensions of the brains were taken with callipers, and are stated in 

 millimetres in Table XIII. 



1 These were removed from the crania and brought to me by the late Mr. C. E. Smith and by Mr. Peffers. 



