94 THE MECHANISM OF LIFE 



cut off entirely from the brain it can regulate and co-ordinate 

 bodily muscular actions and functions. A man, for instance, 

 may " become a father " even when there is a lesion destroying 

 the nervous connection between the brain and the centres in the 

 cord which control the working of the genital organs — that is, 

 those centres are autonomous. Generally speaking, the lower 

 in the evolutionary scale an animal is the greater is the im- 

 portance of the spinal cord relatively to the brain, and vice versa. 

 It is functionally a series of semi-independent ganglia or centres 

 of nervous control. 



The Brain. — It is not certain how many segments have 

 coalesced to form the head of a vertebrate animal. There are 

 ten pairs of cranial nerves, and if we suppose that each of these 

 is, in its origin, to be compared with a spinal nerve, there must 

 be ten segments. But the whole question of head segments is 

 very difficult. Certainly a number of the foremost ones form 

 the head. Goethe thought he could recognise several modified 

 vertebrae in the skull, and this was long afterwards the opinion 

 of anatomists, but we now believe the Goethe- Oken homology to 

 be incorrect. However, the concentration of the great organs of 

 sense in the head and the variety and importance of the movements 

 which are carried out by the eyes and jaws must have led to the 

 concentration of central nervous mechanisms in the neighbour- 

 hood of these organs, and so established the lower brain centres. 



For there are three central nervous systems in the brain — the 

 " lower brain " (oldest), the cerebellum, and the cortex cerebri 

 (the most recent). We must see how these have been evolved. 



The Development of the Nervous System. — The very first 

 thing to be formed in the embryonic development of a vertebrate 

 animal is the central nervous system. A little groove, lying in 

 the future axis of the body, forms in the embryo of a few days 

 old, and then this closes up to form a tube. Soon the foremost 

 (head) end of this tube swells out to form three vesicles, or bulb- 

 like enlargements, and these are the " anlagen " of the brain. 

 The rest of the structure of the animal becomes built up round the 

 primary brain vesicles and spinal cord in a manner which is very 

 difficult to explain, but exceedingly simple and " obvious " when 

 one watches the developmental process in a vertebrate animal. 



Very soon two little lateral vesicles are pushed out from the 

 side walls of the fore-brain, and these become the cerebral 

 hemispheres. The brain thus begins as a hollow organ, and the 



