506 ANIMAL PHYSIOLOGY. 
Fia. 350. 
Fie. 350.—Nervous system of medicinal leech (after Owen). a, double supra-cesophageal 
ganglion connected with rudimentary ocelli (b, b) by nerves ; c, double infra-cesophageal 
ganglionic mass, which is continuous with double ventral cord, having compound ganglia 
at regular intervals. 
Fic. 351.—Nervous system of the common mussel (after Owen). 1, labial ganglia connected 
by a short commissure above and in front of mouth; 8, b, branchial ganglia, connected 
in like manner, and united by long nervous cords (d, d) with labial ganglia; p, bilobed 
pedal ganglion sending branches to the muscular foot (r), and closely united with the 
‘auditory saccules’* (s); h, h’, circum-pallial plexus; y, byssus, by which the animal 
can attach itself to foreign bodies (anchor). 
of the cerebrum, both relative and absolute. In all animals be- 
low the primates (man and the apes) the cerebellum is either 
not at all or but imperfectly covered by the cerebrum, while 
in man, so great is the relative size of the latter, that the 
cerebellum is scarcely visible from above. If we except the 
elephant, in which the brain may reach the weight of ten 
pounds, and the whale with its brain of more than five pounds 
in the largest specimens, the brain of man is even absolutely 
heavier than that of any other animal, which is in great part 
due to the preponderating development of the cerebrum. 
While the cerebral surface is smooth in all the lower verte- 
brates, and but little convoluted until the higher mammals are 
