ARRANGEMENT OF THE NERVOUS SYSTEM 



559 



because they consist of cytoplasm and nuclear material. It stands to 

 reason, however, that their general configuration must be subject to 

 marked variations, because the physiological processes for which they 

 are destined, necessitate an absolute structural adaptation to the 

 conditions existing in different parts of the body. Thus we find that 

 while nerve-cells always present the characteristics of an elongated 

 conductor, they are frequently so highly modified that it becomes 

 difficult to recognize their true nature. Their structural wealth has 

 been brought out more especially in recent years as a result of more 

 advanced methods in fixing and staining J 



It is now commonly believed that neurons are developed from 

 single embryonic cells which are called neuroblasts (Fig. 275). These 

 precursors are compact neuroplasmic 

 masses, possessing a round or oval 

 shape and containing a well-defined 

 nucleus somewhere near the center 

 of their cytoplasm. In the course 

 of time, these apolar cells become 

 pear-shaped and finally send out a 

 process which renders them unipolar 

 and eventually multipolar in char- 

 acter. This theory of His has been 

 modified in more recent years^ by 

 making allowance for the fact that 

 certain fiber paths seem to be de- 

 veloped directly from the neuro- 

 blasts, i.e.y the latter may lose their 

 cellular character entirely and be 

 converted solely into axons. Thus, 

 a number of neuroblasts may be 

 joined together in such a way that 

 a conducting path is produced which 

 is then united with other neuroblasts which have given rise to cell- 

 bodies. 



As has been stated above, the mature neurons present such a wealth of struc- 

 ture that it is impossible to classify them satisfactorily. Many of them, however, 

 present a very characteristic appearance, enabling us to recognize them immedi- 

 ately. Cells of this type are the large pyramidal cells of the motor area of the 

 cerebral cortex, the bipolar cells of the sensory ganglia, the fan-shaped cells of 

 Purkinje of the cerebellum, and others. At all events, any attempt at classifica- 

 tion must take cognizance of the shape and size of the cell-body and of the number, 

 size, and manner of branching of the processes — axis cylinder and dendrites alike. 

 The shape and size of the cell-body vary considerably. In the cerebral cortex, 



^ Among the investigators who have greatly enhanced our knowledge in this 

 regard, might be mentioned Ehrlich (Deutsche med. Wochenschr., xii, 1886, 49), 

 Apathy (Proc. Intern. Zool. Congress, Cambridge, 1898), Golgi (Arch, fisiol., iv, 

 1897), Nissl (Die Neuronenlehre, etc., Jena, 1903), and Ramon Y. Cajal (Hist, de 

 Systeme Nerveux, Paris, 1909). 



2 Baglioni, Zur Analyse der Refiexfunktion, Wiesbaden, 1906. 



Fig. 275.- — Growing Neueoblasts. 



A, Silver method of Cajal; B, Golgi's 



method. (Cajal.) 



