The Structure of the Nervous System
All animals, whose brains are provided with two wrinkled hemispheres, possess the power of muscular movement and of feeling, the faculty of experiencing inner emotions, and, in addition, that of forming ideas, making comparisons and judgments and, in short, of carrying out various acts of intelligence, corresponding to the degrees of development of the hypocephalon.16 (Lamarck, 1809, Zoological Philosophy, III.I, p. 310)
Lamarck divides the human nervous system into five principal parts:
1. Sensorimotor medullar masses situated in each vertebra. Each center sends information via the nerves to stimulate certain muscles and receives information from the nerves that are sensitive to the activity of these same muscles.
2. A spinal cord in which the activity of each medullar mass is transmitted to other medullar masses. This system already exists in insects.
3. The brainstem: The part of the brain that receives the information from the spinal cord. It can therefore centralize information from the body. This primitive brain that provides for vision exists already in insects, spiders, crustaceans, and mollusks.17
4. The limbic system:18 A part of the brain that creates a first centralized coordination of the sensory data to produce some coordinated motor responses of the organism This form of reaction is that of the sentiments,19 the affects, the instincts, and the emotions. Observable in crustaceans and mollusks, the limbic system can be related to the appearance of hearing and then smell.
5. The two lobes of the neocortex, in which an even more complex coordination can take place. It is in this network that Lamarck grounds intelligence. These lobes exist in all of the superior mammals.