3. If folds or convolutions in the surface of the cerebrum indicate intelligence, thoughtfulness, or mind, what do you infer as to the frog's mental condition and power to think?

4. Has the frog brain or "brains"? Explain.

5. Name several things done by an animal's brain.

6. Which of the spinal nerves are specially large? Why should they be large?

7. What advantages are there in the frog's having a dorsal nerve cord instead of a ventral one, as the earthworm has?

What would be the effect of cutting or breaking the dorsal root of a spinal nerve? the ventral root? the entire spinal cord? How do you explain the convulsions of the brainless (beheaded) chicken or frog?

8. What kinds of impulses originate outside of the nerve center, and what kinds in the center?

9. Sensations reach the cord and brain through the dorsal root of the spinal nerve. What kinds of messages travel through the ventral root, and in what direction do they go?

_Suggested drawings._

a. The brain.

b. The spinal cord and its nerves.

c. The nervous system.

d. A diagram of the cord and its nerves, showing the kinds and the directions of the nerve impulses.

The Endoskeleton of the Frog

_Materials._

Prepared frog skeletons mounted in gla.s.s-covered boxes or in other cases suitable for individual study; other vertebrate skeletons for reference.

_Observations._

The pupil should examine his specimen and identify the following structures:--

_A. Skull:--_

1. _Cranium_, or brain case, the central and hinder portion.

2. _Nasal bone_, a triangular bone lying in front of each large opening, or _eye orbit_, and attached to the anterior end of the cranium.

3. _Premaxillaries_, a pair of small bones which form the tip of the nose.

4. _Maxillary_, a slender bone forming the side of the upper jaw.

5. _Dentary_, the bone of the lower jaw, corresponding to the maxillary.

6. _Occipital foramen_, the posterior opening or entrance into the cranium, normally covered dorsally by cartilage and most easily seen in a separate skull.

_B. Vertebral Column:--_

1. _Cervical vertebra_, the first vertebra, supporting the skull.

2. _Dorso-lumbar vertebrae_, vertebrae with small lateral processes.

3. _Sacrum_, the ninth vertebra, bearing extra long lateral processes or "arms."

4. _Urostyle_, the last vertebra, lying in the median line, a long, slender, blade-like bone, really formed by the fusion of several vertebrae.

_C. Vertebra:--_

1. _Centrum_, the solid, nearly circular portion.

2. _Lateral process_, one of the paired projections, extending outward on either side.

3. _Neural spine_, the single projection, extending toward the dorsal side.

4. _Neural arch_, formed largely by a connection between the lateral process and the neural spine. The arch above and the centrum below inclose the _neural ca.n.a.l_.

_D. Pectoral, or Shoulder, Girdle:--_

1. _Sternum_, the "breastbone," extending along the median ventral line.

2. _Coracoid_, a heavy bone extending from the sternum sidewise to support the arm.

3. _Clavicle_, the "collar bone," a light bone in front of the coracoid.

4. _Scapula_, the "shoulder blade," a broad, thin bone which arches around to the dorsal side.

_E. Pelvic, or Hip, Girdle:--_

1. _Ilium_, a long, slender, curved bone, with its mate uniting to form an inverted "wishbone"; it is joined to the sacrum.

2. _Pelvis_, the region of the socket at the junction of the ilia.

_F. Fore Leg, or Arm:--_

1. _Humerus_, the single bone of the upper arm.

2. _Radius_, the bone of the lower arm on the thumb side; in the frog united to the ulna.

3. _Ulna_, the bone of the forearm opposite the radius.

4. _Carpals_, small bones of the wrist.