SCIENCE 



NEW YORK, JUNE 30, 1893. 



THE EVOLUTION OF CONSCIOUSNESS AND OF THE 

 CORTEX. 



BY C. L. HERRICK, PROFESSOR OF BIOLOGY IN DENISON UNIVERSITY, 

 GRANVILLE, OHIO. 



It would be difficult to find an illustration of the mutual inter- 

 dependence of the biological sciences more striking than that 

 afforded by the recent contributions from morphology, embryology, 

 physiology, and pathology to our knowledge of the significance of 

 the cerebrum. 



It has been customary to scout at any substantial contribution 

 to psychology from the experimental sciences, and even now, 

 when so much attention is given to psycho-physics or physiologi- 

 cal psychology, far too little use is made of the data of modern 

 embryology and histology. It should be apparent to all who are 

 not a priori convinced that no relation exists between conscious- 

 ness and the nervous system, that no satisfactory super-structure 

 can be reared upon any other foundation than that afforded by a 

 minute study of the structure and function of the brain. 



It is the great triumph of modern embryological histology, with 

 Professor His as its leader, to have discovered the essential simi- 

 larity of all nervous elements. What Schwann did for biology at 

 large by means of the cell theory, Professor His has done for 

 neurology through his theory of the neuroblast and its supple- 

 ment, the "neuron theory." 



It was inevitable that we should soon recognize the essential 

 similarity in origin and structure of all nervous cells. The present 

 writer has insisted for some years that the entire fabric of the 

 nervous system, with the exception of a few connective and 

 nutritive elements of secondary nature, is woven by the inter- 

 blending of neurons of similar character. All such neurons are 

 formed from the epiblast or its derivatives (the perplexing rela- 

 tions of the sympathic system aside). Each neuron arises from 

 neuroblasts or formative cells springing from the ectal surface 

 (ventricular surface by invagination in the case of the axial ner- 

 vous system), and, after migration to its definitive site, takes on its 

 distinctive character. It has been attempted to show that these 

 are all transitions between the neuroblast and the wonderful 

 variety of nervous elements. The nerves, whether springing from 

 a special ganglion or from the neuraxis itself, are formed, in our 

 view, from the moniliform union of neuroblasts, whose nuclei, 

 when they have served their purpose in forming the fibre, become 

 separated to form the nuclei of the sheath. The recent researches 

 in nerve degeneration and histogenesis all favor this view. 



Besides those elements which at once become transformed into 

 the definitive nerve cells, we believe there are intermediate condi- 

 tions or "reserves," which may subsequently be called into 

 function. Upon this view there is a continuous intercallation of 

 nervous elements going on — a process much more rapid during 

 youth. From the same standpoint it seems probable that there 

 are numerous proliferating stations, where such neurons are con- 

 tinually forming. In the cerebellum and medulla, and even in 

 the cerebrum itself, there are such loci of rapid development. No 

 exception has so far been encountered to the law that the neurons 

 of the central system all spring directly or indirectly from the 

 ventricular surface. 



An attentive comparative study of the various groups of verte- 

 brates shows that the development of the various parts of the 

 brain obeys simple and readily discoverable laws, which, when 

 recognized, are as self-evident as the gastraea formation of the 

 embryo. The modifications of the brain-tube, from its primitive 



uniformity to the wonderful complexity to which it attains in 

 man, form a consecutive series without any complexing hiatus. 



Interest attaches particularly to the cerebrum, by reason of its 

 preeminent position, as the latest structural modification, and its 

 close relation to the phenomena of consciousness. Since Riick- 

 bard showed that in the fish the roof or cortex of the cerebrum is 

 wanting, or rather represented by a non-nervous membranous 

 pallium, considerable modifications in our conceptions of the 

 sphere of consciousness have been rendered necessary. Remark- 

 able experiments show that the whole cerebrum may be removed 

 without making any noticeable diGference in the habits and activi- 

 ties of the fish (save in the case of those functions associated with 

 smell). The writer has studied the axial lobe of the cerebrum of 

 fishes and described numerous distinct cell-clusters and tracts 

 which had hitherto been overlooked. He suggested that the un- 

 dififerentiated prototypes of the cell-masses, which in higher verte- 

 brates occupy the cortex, are, in this case, retained in the axial 

 lobes. It was shown that the centres for the sense of smell are 

 highly and specially developed, and are connected by strong and 

 distinct tracts with the olfactory organs. It was even ventured 

 to locate a homologue of the hippocampus upon the basis of the 

 tracts. This procedure was evidently regarded by some as rash, 

 but has been amply justified by subsequent developments. 



In reptiles we located the olfactory centre or hippocampal lobe 

 in a large part of the cortex, which is closely associated with a 

 curiously modified part of the axial lobe. We suggested that the 

 cortical elements arise as proliferations from the axial lobe, which 

 push out into the thin cortical walls or pallium. In the Leuckart 

 Festschrift it was shown that the preplexus in amphibia is analo- 

 gous in early position and structure to the pallium of fishes. 



In a recent number of the Anatomischer Anzeiger Dr. Edinger, 

 who is perhaps the ablest living comparative neurologist, works 

 out this form of the solution of this problem in detail with respect 

 to the olfactory and hippocampus. Accepting the suggestion of 

 proliferation from the axial lobe, he shows that the earliest cortex 

 to be formed is that which, in higher vertebrates, is termed 

 ammonshorn or hippocampus. In other words, consciousness 

 first intervenes in the construction of data from the olfactory 

 sense. This suggestion is enforced by the data of comparative 

 morphology. The olfactory is the most primitive of the special 

 sense-organs, and is most closely associated with the cerebrum. 



Several years ago we proposed the theory that consciousness 

 must have appeared very late in the evolution of psychical func- 

 tions ; the higher expressions of this faculty, such as reflection, 

 being among the latest endowments of the race. It was shown 

 that such a view would give us less concern in the bloodthirsty 

 procession of ferocious animals which have reddened every page 

 of geologic history. When the greatest diameter of the nerve- 

 tube was in the pelvic region, it was unnecessary to predicate 

 consciousness as a prerequisite to the simple avocations of the 

 animal. 



We believe that, under the law of natural selection, con- 

 sciousness could only appear when the arena was opened for its 

 serviceable exercise. 



Remarkable confirmation of the comparatively accessory status 

 of consciousness has been obtained from two such different sources 

 as the study of hypnotism and experimental psychology. In a 

 most interesting paper printed in the June number of the Journal 

 of Comparative Neurology Dr. Edinger describes the results of an 

 examination of the brain of a dog, from which Professor Goltz 

 had removed the entirt cerebrum on both sides. 



The dog lived eighteen months, but, contrary to the predictions 

 of the sceptical, the cerebrum proved to be all but entirely re- 

 moved. The special senses were not destroyed except smell. Lo- 

 comotion was not impaired, and general sensation was intact. 



