72 KANSAS ACADEMY OF SCIENCE. 



connective tissue framework of the brain. These cells shrink and collapse in the 

 center; but in lower brains, at least, our sections seem to show that they retain 

 their continuity from the ventricular to the outer surface of the brain and cord. 

 Between these cells (spongioblasts) there appear, according to Professor His, at an 

 early stage, isolated nuclei, probably derived from the nuclei of the spongioblasts. 

 These so-called neuroblasts have remarkable powers of motion, and migrate, some- 

 times in large numbers, to adjacent parts of the brain or spinal cord, as the case 

 may be. From these neuroblasts are derived all nerve cells and fibers; from the 

 spongioblasts, the connective tissue elements of the brain. In a subsequent paper, 

 on the development of the human spinal cord, Professor His applies these principles 

 in detail, tracing the origin of the various nuclei and tracts of the cord and medulla. 

 For example, he shows how the olivary bodies are produced by a pouching off of a 

 portion of the fourth ventricle on each side, and the migration of its cells to their 

 positions near the ventral surface of the medulla. 



This principle finds application elsewhere frequently in the brain. The cortical 

 cells of the cerebrum, for example, are not all developed where they lie, but several 

 well-marked areas of proliferation have been found from which cells pass to their 

 respective sites. Thus, in the axial lobe, or brain base, of several groups of reptiles, 

 cells are very rapidly formed, which seem to pass to the adjacent regions of the 

 cortex.* Mr. Turner finds the same relations in birds.! In the fish brain, the en- 

 tire cortex cerebri is represented by an epithelial membrane with no nervous organs, 

 and these areas of proliferation in the axial lobes perform the whole function of the 

 cerebrum. 



We pass now to a series of investigations which may truly be called revolution- 

 ary. Since the time of Spinoza, it has been the custom to consider the central 

 nervous system as analogous to a central battery system, toward which messages 

 are sent along the various nerve fibers from the sensory organs, and from which 

 similar messages are sent to the muscles and other instruments of volition. The 

 transmission of such messages implies the physical union of the axis cylinder of a 

 nerve fiber with a cell of the sensory or motor organ in question, on the one hand; 

 on the other hand, physical union of the same fiber with some cell in the central 

 nervous system, these cells being so connected as to insure the continuity of the 

 motor with the sensory nerve systems. Upon this theory Professor Meynert based 

 his now famous scheme of projection systems, tracing the path of a sensory im- 

 pulse from cell to fiber and from fiber to cell, until it reaches the psychical cells of 

 thought, in the cortex of the frontal lobes of the cerebrum, thence to the motor 

 cells, the striatum, and the motor nerve. 



But now Professor Golgi and his followers come forward and affirm that no 

 nerve fiber is in direct physical continuity with a nerve cell at each end. Nervous 

 excitation is not a mere transmission of an impulse through a suitable conductor, 

 for the circuit is broken at many points. By processes of impregnation of the 

 tissues with chromate of silver, now familiar to every histologist, they claim to 

 make every finest nerve fibrilla stand out with diagrammatic distinctness, and that 

 when thus differentiated every cell is found to be quite independent of every other 

 cell. From each cell there pass two kinds of processes: First, an axis-cylinder proc- 

 ess, which passes into the nerve fiber; and second, the protoplasmic processes, 

 smaller and much branched, said by Golgi to be nutritive in function, but by others 

 regarded as undoubtedly nervous. This, then, is our neurological unit, from which 

 all nervous tissues are elaborated, a cell with its processes of two kinds. The axis- 



*Jour. Comp. Neurol., I, p. 17. 

 tibid., p. 71. 



