124 ^- ^- HIMWICH VOL. 4 (1950) 



To climb the phyletic ladder from our remotest ancestors through the fish, am- 

 phibia, reptiles and mammals, would entail a tremendous volume of description, which 

 is not the point of this contribution. The general trend of this process of cephalization, 

 or concentration of neural functions in the oral end of the animal, may be described 

 briefly: as far back as the fish, brain is divided into five portions as it is in man, but in 

 the fish and amphibia the chief site of integration for sensory and motor impulses lies 

 in the midbrain. In these species the highest portion of the brain consists chiefly of the 

 olfactory bulb, and the cerebral cortex which becomes all-important in man, is repre- 

 sented only by a thin layer of cells. On further ascending the phyletic scale to reptiles 

 and birds as well as mammals, the subcortical structures immediately anterior to the 

 midbrain become more prominent, as the organism achieves greater coordinating 

 control. Lastly, the cerebral cortex, though getting off to a late start, gradually attains 

 more complexity of structure and diversity of function until in the lower mammals it 

 surpasses all other regions, and in the primates, especially in man, forms the largest and 

 most comple:^ part of the cerebral tissue. As this process of phylogeny is carried on 

 from one species to another, no part of the neuraxis is scrapped, but each older part, 

 in turn, comes under the influence of a later developed portion, which not only possesses 

 finer discrimination and analj'sers but also plays a role in determining the motor 

 expression of the older areas. 



Though the brain of man as we see it today looks like a static structure, when it is 

 examined more closely in the light of the phyletic conception, we see that it has come 

 to its present construction as a result of a long series of accretions, beginning with the 

 spinal cord and medulla oblongata and spreading in a cephalad direction, layer upon 

 layer, until the cerebral hemispheres form the greatest mass of the brain. It is not to 

 be supposed that each level is independent of its predecessors, but rather that it exists 

 with a specific relation, both anatomically and physiologically, to the phyletically 

 older portions'*^. Owing to this relation, the central nervous system may function as a 

 unit, but a unity which is brought to a higher plane of integration with each successive 

 step. The human brain is undoubtedly the latest arrangement of the central nervous 

 system, but not necessarily the final one. 



Sir Charles Sherrington^^ has expressed vividly Hughlings Jackson's con- 

 ception. "That leading end, the head, has receiving stations signalling from things at 

 a distance, things which the animal in its forward movement will next meet. A shell 

 of its immediate future surrounds the animal's head. The nerve-nets in the head arc 

 therefore busy with signals from a shell of the outside world which the animal is about 

 to enter and experience. The brain has thus arisen where signalling is busiest and is 

 fraught most with the germ of futurity. Small wonder then that the brain plays a great 

 role in the motor management of the muscle. Nerve management of muscle resolves 

 itself largely into management of nerve by nerve, especially by brain, more and more 

 so as evolution proceeds. With no greater equipment of muscle the superimposed 

 amount of nerve becomes greater and greater; each new nerve-growth seems to entail 

 further nerve-growth. Fresh organization roofs over prior organization. Brain is an 

 example. 'So on our heels a fresh perfection treads'. But were it a government ofiice we 

 might be suspicious. This brain of ours is a perfect excrescence although our endowment 

 of muscle remains but moderate". 



References p. 125. 



