140 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”Fig. 7.8. Vertebrate brains in evolution: (c) cerebrum (brain proper); (ce) cerebellum
(for balance); (ob) olfactory bulb (for smell). Drawings not to scale. Note the relative
expansion of the cerebrum and the shrinking of olfactory bulb in the human brain. [See
Note 34: Romer, Parsons; permission Cengage Learning.]
The brain of a vertebrate is encased in a protective bony cavity.
This provides the brain, basically a soft, gelatinous mass of matter vul-
nerable to physical damage, a chance to develop delicate structures nec-
essary for higher mental functions. But the skull also sets an upper limit
for the size of the brain to evolve. The maximum skull size, on the other
hand, is constrained by the pelvic opening of the mother through which
a newborn must be delivered. The brain may appear left-right symmetri-
cal, but in reality some division of labor exists. For example, most people
have their language center located on the left side.
The spinal cord is the downward extension of the brain that goes
through the entire length of the body. It comprises two major systems,
the ascending tracts which convey sensory information from the body
to the brain, and the descending tracts made up of motor neurons that