1. The co-operation of the parts of the body may be illustrated as follows: a. Feign a blow at a person's eye; the lids will close involuntarily, even if he be told beforehand that he is not to be actually struck.

b. Count a boy's pulse and breathing while he is sitting quietly, then let him run a hundred yards at full speed, and immediately afterwards again count pulse and breathing movements. Both will be found accelerated; the breathing, to carry off from the blood the carbon dioxide given it by the working muscles, and to bring in new oxygen to replace the large amount used by the working muscles; the heart-beat, to renew more rapidly the blood-flow through the muscles.

c. Tickle the inside of the nose with a feather. This, in itself does not interfere with the breathing muscles; but their action will be almost at once so changed as to produce a sneeze, tending to clear and protect the nose.

2. Kill a frog with ether (note, p. 86); open its abdomen and remove the viscera. At the back of the abdominal cavity will be seen a bundle of white cords (nerve-trunks) passing back to each leg. They soon unite into one main stem (the sciatic nerve), which may be easily dissected along its course until it ends in fine branches in the hind limb.

3. Kill a frog and expose the origin of the sciatic nerve as above. With stout scissors then cut away bit by bit, and very carefully, the bodies of the vertebra (which will be seen projecting in the middle line at the back of the abdominal cavity) until the neural canal is laid open and the spinal cord exposed. You will probably fail the first time, but on the second attempt succeed in doing this without cutting the nerve trunks as they pass between the vertebra to Join the spinal cord. On the specimen thus prepared the origin of the nerves from the spinal cord, and their division into anterior and posterior (ventral and dorsal) roots before they join the cord can be demonstrated, also the ganglionic enlargements on the posterior roots.

4 The general form, the cervical and lumbar enlargements, etc., of the spinal cord may be shown on a frog. Having killed the animal, remove the skin and muscles on the dorsal side of the spinal column. With great care cut away the upper two thirds of the neural arches of the vertebrę. Then remove the upper half of the skull cavity. Gently raising piece by piece the exposed brain and spinal cord, divide the nerves which spring from them and lift out the whole cerebro-spinal centre and place it in alcohol for twenty-four hours. Demonstrate the origin of nerves from both brain and cord, the union of the brain and cord, etc. etc. The specimen may be preserved in alcohol for future use.

5. A frog's brain differs in many important points from that of man, as in the very small cerebellum, the comparatively small cerebral hemispheres, the comparatively large mid-brain and the absence of convolutions. To demonstrate the main anatomical features of the brain that of a mammal is necessary.

a. Obtain a fresh calf's or sheep's head from a butcher. Dissect away the skin and muscles covering the cranium. Then with a small saw very carefully divide the bones in a circular direction, so as to cut off those of the crown of the head. Next carefully remove the loosened bones of the top of the skull, tearing them away from the dura mater lining them. So far the specimen may be prepared previous to the meeting of the class.

b. To the class demonstrate the tough dura mater enveloping the brain; then cut it away, noting the processes which it sends between the two cerebral hemispheres and between cerebellum and cerebral hemispheres. Then cut the membrane away.

e. Note its glistening inner surface, due to the arachnoid lining it; the pia mater full of blood-vessels and closely attached to the brain; the glistening arachnoid layer covering the exterior of the pia mater. Then put the specimen aside in alcohol for a day or two. This will harden the brain substance.

d. When the brain has become somewhat hardened dissect away the pia mater on one side. Show the cerebral hemispheres and their surface convolutions, the cerebellum and its foldings, the medulla oblongata beneath the cerebellum.

e. With bone forceps cut away the remainder of the sides and roof of the skull. Then raise the brain in front, and cutting through the vessels, nerves, etc., which attach it to the base of the skull, entirely remove it from the skull cavity. On it demonstrate the cerebral hemispheres (which overlap the cerebellum much less than in man), cerebellum, mid-brain, etc.

f. Attached to the base of the brain will be found the stumps of some of the cranial nerves, though most of these will have been entirely torn off unless the dissector has some technical skill. The optic commissure, with the optic tracts leading to it and the stumps of the optic nerves leading from it, will almost certainly be found.

g. Make sections across the brain in different directions to see the gray matter spread over most of its surface, and the nodules of gray matter imbedded in its interior.