Technical Note.—In a specimen which has been macerated for some time in 20% nitric acid dissect out the nervous system. Place the specimen in a pan ventral side uppermost and pin out. Carefully pick away the vertebræ and the roof of the mouth-cavity, thereby exposing the central nervous system, which will appear light yellow.
Examine the brain. In front of the true brain are the olfactory lobes, the nervous centre for the sense of smell. The brain itself is composed of several parts. The anterior portion consists of two elongated parts, the cerebral hemispheres; just back of these are the optic lobes or midbrain, consisting of two short lobes, which are followed by the small cerebellum, which in turn is followed by a long part, the medulla oblongata, which runs imperceptibly into the long dorsal nerve, the spinal cord. Note the large optic nerves running out to each eye. How far backward does the spinal cord extend? Note the many pairs of nerves given off from the brain and spinal cord. These nerves branch and subdivide until they end in very fine fibres. Some end in the muscle-fibres, and through them the central nervous system innervates the muscles. These are motor endings. Still others pass to the surface and receive impressions from the outside. These last are sensory endings. Note that the spinal nerves arise from the spinal cord by two roots, an anterior or ventral, and a posterior or dorsal root. Trace the principal spinal nerves to the body-parts innervated by them. These nerves are numbered as first, second, etc., according to the number of the vertebræ (counting from the head backward) from behind which they arise.
[CHAPTER III]
THE STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY
Organs and functions.—The body of the toad is composed of various parts, such as the lungs, the heart, the muscles, the eyes, the stomach, and others. The life of the toad consists of the performance by it of various processes, such as breathing, digesting food, circulating blood, moving, seeing, and others. These various processes are performed by the various parts of the body. The parts of the body are called organs, and the processes (or work) they perform are called their functions. The lungs are the principal organs for the function of breathing; the heart, arteries and veins are the organs which have for their function the circulation of the blood; the principal organ concerned in the digestion of food is the alimentary canal, the function of seeing is performed by the organs of sight, the eyes, and so one might continue the catalogue of all the organs of the body and of all the functions performed by the animal.
The animal body a machine.—The whole body of the toad is a machine composed of various parts, each part with its special work or business to do, but all depending on one another and all co-operating to accomplish the total work of living. The locomotive engine is a machine similarly composed of various parts, each part with its special work or function, and all the parts depending on one another and so working together as to perform satisfactorily the work for which the locomotive engine is intended. An important difference between the locomotive engine and the toad's body is that one is a lifeless machine and the other a living machine. But there is a real similarity between the two in that both are composed of special parts, each part performing a special kind of work or function, and all the parts and functions so fitted together as to form a complex machine which successfully accomplishes the work for which it is intended. And this similarity is one which should help make plain the fundamental fact of animal structure and physiology, namely, the division of the body into numerous parts or organs, and the division of the total work of living into various processes which are the special work or functions of the various organs.
The essential functions or life-processes.—The toad has a great many different special parts in its body. Its body is very complex. It performs a great many different functions, that is, does a great many different things in its living. And the structure and life of most of the other animals with which we are familiar are similarly complex: a fish, or a rabbit, or a bird has a body composed of many different parts, and is capable of doing many different things. Are all animals similarly complex in structure, and capable of doing such a great variety of things? We shall find that the answer to this question is No. There are many animals in which the body is composed of but a few parts, and whose life includes the performance of fewer functions or processes than in the case of the toad. There are many animals which have no eyes nor ears nor other organs of special sense. There are animals without legs or other special organs of locomotion; some animals have no blood and hence no heart nor arteries and veins. But in the life of every animal there are certain processes which must be performed, and the body must be so arranged or composed as to be capable of performing these necessary life-processes. All animals take food, digest it, and assimilate it, that is, convert it into new body substance; all animals take in oxygen and give off carbonic acid gas; all animals have the power of movement or motion (not necessarily locomotion); all animals have the power of sensation, that is, can feel; all animals can reproduce themselves, that is, produce young. These are the necessary life-processes. It is evident that the toad could still live if it had no eyes. Seeing is not one of the necessary functions or processes of life. Nor is hearing, nor is leaping, nor are many of the things which the toad can do; and animals can exist, and do exist, without any of those organs which enable the toad to see and hear and leap. But the body of any animal must be capable of performing the few essential processes which are necessary to animal life. How surprisingly simple such a body can be will be later discovered. But in most animals the body is a complicated object, and is able to do many things which are accessory to the really essential life-processes, and which make its life complex and elaborate.