_B. Spinal cord_, extending along the spine, giving rise to nerves. _C. Spinal nerves_, ten pairs of nerves which are connected with the cord through dorsal (sensory) and ventral (motor) roots, and which penetrate the body and its appendages. The first enters the neck; the second and third join and enter the arm; the fourth to sixth penetrate the skin and muscles of the trunk; the seventh to ninth join by a plexus to form the sciatic nerve which supplies the leg, and the tenth enters the posterior portion of the body. _Questions._ 1. Which lobes of the brain are paired? Give at least two possible causes or reasons for their double structure. 2. The optic lobes are connected with the eyes. Compare their size with that of the other parts. Of how much use do you think they are to the frog? 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 glass-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.