Back to the Main Page Updates from the crewHere we areMission detailsWe keep busySee us in actionMission emblemsPlaces to goNASA's Columbia sitePrevious SLS MissionsNASA's Spacelab siteLaunch detailsAsk us a question

MIT Encyclopedia of the Cognitive Sciences, MIT Press, Cambridge, Mass., 1998
by Javier DeFelipe
Instituto Cajal (CSIC), Avenida Dr. Arce, 37 (28002-Madrid, Spain)
Ph:  (34-1) 585 4735; Fax: (34-1) 585 4754
reprinted with permission from the author
Santiago Ramón y Cajal (1852-1934) was one of the most outstanding neuroscientists of all time.  He was born in Petilla de Aragón, a small village in the North of Spain. He studied medicine in the Faculty of Medicine in Zaragoza.  In 1883 Cajal was appointed to the chair of Descriptive and General Anatomy at the University of Valencia. In 1887 he moved to the University of Barcelona where he was appointed to the chair of Histology and Pathological Anatomy.  Then Cajal moved, until his retirement, to the University of Madrid where he was appointed to the chair of Histology and Pathological Anatomy.  Dr. Cajal received numerous prizes, honorary degrees and distinctions, among the most important being the Nobel Prize for Physiology or Medicine in 1906.  To describe the work of Cajal is rather a difficult task, because, unlike other great scientists, he is not known for one discovery only, but for his many and important contributions to our knowledge of the organization of the nervous system.  Those readers interested in his life should consult his autobiography (Cajal, 1917), where there is also a brief description of his main discoveries and theoretical ideas. 

The detailed study of the nervous system began in the middle of last century.  Before Cajal's discoveries, very little was known about the neuronal elements of the nervous system, and the connections between its different parts were purely speculative.  The origin of nerve fibers was a mystery, and it was speculated that they arose from the gray matter independently of the nerve cells (neurons).  This lack of knowledge was due mainly to the fact that appropriate methods for visualizing neurons were not available; the early methods of staining only permitted the visualization of neuronal cell bodies, a small portion of their proximal processes, and some isolated and rather poorly-stained fibers.  It was in 1873 when the method of Camillo Golgi (1843-1926) appeared; for the first time, neurons were readily observed in histological preparations with all their parts: soma, dendrites and axon.  Furthermore, Golgi-stained cells displayed the finest morphological details with an extraordinary elegance, which led to the characterization and classification of neurons, as well as to the study of their possible connections.  Due to this technical advance, Golgi shared the Nobel Prize with Cajal. 

Cajal was not introduced to a scientific career under the direction of any scientist, as then usually occurred with most scientists, but rather he became a prominent neurohistologist on his own.  The career of Cajal can be divided into three major phases (DeFelipe and Jones, 1991): 

  • The first phase extended from the beginning in 1877 until 1887, when he was introduced to the method of Golgi. During this period he published a variety of histological and microbiological studies, but they were of little significance.
  • The second phase (1887-1903) was characterized by a very productive research activity, in which he exploited the Golgi method in order to describe in detail almost every part of the central nervous system.  These descriptions were so accurate that his classic book Histologie (Cajal, 1909, 1911), in which these studies are summarized, is still a reference book in all laboratories of neuroscience. Also, during the first few years of this phase, Cajal found much evidence in favor of the Neuron Doctrine, which was in contrast to the other more commonly accepted principle of the Reticular Theory.  The Neuron Doctrine, which is the fundamental organizational and functional principle of the nervous system, states that the neuron is the anatomical, physiological, genetic and metabolic unit of the nervous system, whereas for the Reticular Theory the nervous system consisted of a diffuse nerve network formed by the anastomosing branches of nerve cell processes (either both dendritic and axonal, or only axonal), with the cell somata having mostly a nourishing role (for review, see Shepherd, 1991; Jones, 1994).
  • The third phase began in 1903, with his discovery of the reduced silver nitrate method, and ended with his death in 1934; this period was also very productive and was devoted mainly to the theme of traumatic degeneration and regeneration of the nervous system. He published numerous scientific papers about this subject which were of great relevance, and which were summarized in another classic book, Degeneration and Regeneration (Cajal, 1913-1914). During this phase, Cajal also published some important papers on the structure of the retina and optic centers of invertebrates.
Interestingly, Golgi, as well as most neurologists, neuroanatomists and neurohistologists of that time, was a fervent believer in the reticular theory of nerve continuity.  However, for Cajal the Neuron Doctrine was crystal clear.  Nevertheless, microphotography was not well-developed at that time, and virtually the only way to illustrate observations was by means of drawings, which were open to skepticism (DeFelipe and Jones, 1992).  Some drawings of Cajal were considered as artistic interpretations rather than accurate copies of his preparations.  However, examination of the Cajal's preparations, housed in the Cajal Museum at the Cajal Institute, proves the exactness of his drawings (DeFelipe and Jones, 1988, 1992).  Although Cajal had the same microscopes and produced similar histological preparations with comparable quality of staining as the majority of the neurohistologists of his time, he saw differently than they did.  This was the genius of Cajal. 


Cajal, S.R., 1909, 1910, Histologie du système nerveux de l'homme et des vertébrés. (Translated by L. Azoulay). Paris: Maloine. There is an English translation: Histology of the nervous system of man and vertebrates (translated by N. Swanson and L.W. Swanson), New York: Oxford University Press, 1995). 

Cajal, S.R., 1913-1914, Estudios sobre la degeneración y regeneración del sistema nervioso. Madrid: Moya. There is an English translation: Degeneration and regeneration of the nervous system (translated and edited by Raoul M. May), London: Oxford University Press, 1928). Reprinted and edited with additional translations by DeFelipe, J. and E.G. Jones: Cajal's degeneration and regeneration of the nervous system. New York: Oxford University Press, 1991). 

Cajal, S.R., 1917, Recuerdos de mi vida, Vol.2: Historia de mi labor científica. Madrid: Moya. There is an English translation: Recollections of my life (translated by E.H. Craigie with the assitence of J. Cano), Philadelphia: American Philosophical Society, 1937). Reprinted Cambridge, Mass.: MIT Press, 1989. 

DeFelipe, J. and E.G. Jones,.1988, Cajal on the cerebral cortex. New York: Oxford University Press. 

DeFelipe, J. and E.G. Jones, 1991, Cajal's degeneration and regeneration of the nervous  system. New York: Oxford University Press. 

DeFelipe, J. and E.G. Jones, 1992, Santiago Ramón y Cajal and methods in neurohistology. Trends in Neuroscience 15: 237-246. 

Jones, E.G, 1994, The neuron doctrine. Journal of History of Neuroscience 3: 3-20. 

Shepherd, G.M., 1991, Foundations of the neuron doctrine. New York: Oxford University Press.