458 - Multicellular Animals, Especially Man 



birds and mammals. This effect is very im- 

 portant, because facilitation may underlie 

 the phenomena of habit, memory, and learn- 

 ing. In fact, all conditioned reflexes, which 

 enable an animal to modify behavior in rela- 

 tion to past experience, appear to be depend- 

 ent upon the facilitation effect. 



If fatigue is avoided, facilitation becomes 

 more and more pronounced as the synapses 

 of a given arc continue to be used, until a 

 certain maximum is reached. The degree of 

 repetition required to reach this maximum 

 varies widely from species to species, and 

 from individual to individual in the same 

 species. Moreover, facilitation tends to en- 

 dure for considerable time — sometimes 

 throughout the life of the individual. The 

 lading of facilitation — and this is the under- 

 lying basis of forgetting and loss of habit — 

 usually proceeds at a gradual pace, during 

 periods when there is a prolonged disuse of 

 the synapses concerned with a particular re- 

 sponse. However, there are many facets to 

 the problem of facilitation that still remain 

 obscure. 



THE NERVOUS SYSTEM OF MAN AND 

 OTHER VERTEBRATES 



The vertebrate nervous system is far more 

 complex than that of invertebrate animals, 

 although no fundamental dillerences have 

 been found between the systems. 



The Central Nervous System. In vertebrate 

 animals, the central nervous system consists 

 of the brain and spinal cord, which form a 

 thick-walled tubular mass of nerve tissue 

 extending lengthwise in the mid-line of the 

 dorsal bod) wall. At its anterior extremity 

 the nerve tube is enlarged and modified, 

 forming the brain; but most of the tube 

 forms the spinal cord, which has a relatively 

 simple cylindrical form (Fig. 25-10). The 

 brain lies within a strong (usually bony) 

 encasement, called the cranium; and the 

 spinal cord is surrounded by the segments of 

 the vertebral column. 



The form of the spinal cord does not vary 



OLFACTORY 

 LOBE 



CEREBRUM 



THALAMUS 



MIDBRAIN 

 (OPTIC LOBE) 



CEREBELLUM 

 MEDULLA 



SPINAL CORD 



Fig. 25-10. Brain and spinal cord of a frog. Cranial 

 nerves are indicated by Roman numerals, l-X; spinal 

 nerves, by Arabic numbers, 1-10. 



very much in different vertebrates, except in 

 size and length (Fig. 25-11). The brain, how- 

 ever, becomes proportionately larger and 

 displays an increasing complexity of form 

 and function in higher vertebrates, especially 

 in man and other mammals (Fig. 25-12). 



Structure of the Spinal Cord and Brain. A sec- 

 tion of the spinal cord (Fig. 25-13) reveals 

 two regions: (1) an outer mass of white mat- 

 ter, composed mainly of axons running 

 lengthwise in the cord; and (2) an inner 



