The cerebellum : its functions, diseases and encephalic interrelations / Charles K. Mills, M.D., LL.D., Emeritus Professor of neurology in the Medical School of the University of Pennsylvania, Philadelphia.

  • Mills, Charles K. (Charles Karsner), 1845-1931
Date:
[1928]
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Licence: In copyright

Credit: The cerebellum : its functions, diseases and encephalic interrelations / Charles K. Mills, M.D., LL.D., Emeritus Professor of neurology in the Medical School of the University of Pennsylvania, Philadelphia. Source: Wellcome Collection.

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    longitudinally along the body of the fish; hence the name “lateral line” system. On the head, branches of this system extend over the supra-orbital, infra-orbital and mandibular areas. Occasionally, some of these nerve hills, especially on the head, are exposed or merely protected in shallow grooves, though this is the exception; the usual and almost universal condition is one in which the nerve hills are included in tubes which are in every way analogous to the semicircular canals. These tubes are formed by the grooving or involution of epidermal structures and the coalescence of the opposing edges with the resulting formation of canals. It is exceedingly probable that the semicircular canals of the labyrinth were originally portions of the lateral line system which became deeply submerged and gradually acquired a position below the base of the skull. This becomes the more probable when one reflects that the petrous portion of the temporal bone in which the semicircular canals are embedded is really not a part of the vertebrate endoskeleton, but is in reality ectoskeletel in origin; a surviving portion of an ancient ectoskeletal structure which in due course became incorporated with the base of the skull. We should call to mind also the fact that not all of the fishes have their semicircular canals, for the myxines—the hag fishes—have but one and the lamprey eels have only two. The function of the nerve hills of the lateral line system of fishes is probably to act as ectoroceptors for coarse waves or vibrations in the water; perhaps they have other functions as well, such as the transmission of the varying pressures of different depths; but it would seem that the transmission of the impacts of coarse waves or vibrations is their principal or outstanding function, while it would be logical to conclude that the reception of impacts of a higher rate of vibration is the function of the semicircular canals. Coupled with the reception of coarse vibrations by the lateral lines there is neces¬ sarily a difference in the intensity with which the impacts impinge on the various portions of the lateral line system dependent on the direction from which the waves come. Intensity of impression and direction appear, therefore, to be indis¬ solubly connected. In other words, the facts justify the conclusion that the lateral line system is intimately related to the spatial orientation of the animal and to its adjustment to its environment. Here, it seems to me, we have indicated the solution of the function of the semicircular canals. In the evolution of the semicircular canals from a primitive lateral line system, the function of the perception of environmental relations has been definitely preserved and in time—in the course of evolution—specialized so that this function has come to be repre¬ sented by three canals bearing to each other the relations of the three classic planes of space; necessarily at right angles to each other. Thus are fulfilled in a striking manner all of the possible demands of the function of spatial orientation. In keeping with the view that the lateral line system of fishes and the semi¬ circular canals are primitively a part of one structure is the fact that both the nerves from the lateral lines and from the semicircular canals terminate in the same portion of the medulla; namely, in the acusticolateral area. Further, there is every reason to believe that in this portion of the medulla other impacts arising from the proprioceptors in the muscles, bones and joints, from the interoceptors and proprioceptors of the various viscera are likewise received. Another interesting fact now presents itself and that is that the cerebellum is developed from this same acusticolateral area. As the demands of adjustment to the environment increase, this area increases in size and complexity. When the aquatic vertebrate becomes terrestrial, when it begins to employ its extremities in crawling and in other forms of terrestrial locomotion—in running, climbing, prehension, erect walking and, in birds, of flying—the increasing intricacies of movement are accompanied by an increase in the size and complexity of the cerebellum.