NERVE SUPPLY OF MAMMALIAN PINEAL ORGAN 407 



Sympathetic system j Afferent nerve-fibres to the pineal body. 



[Efferent nerve-fibres from the pineal body. 



Also one might expect to find in connection with these fibres sensory 

 or receptive cells and ganglion cells, the latter giving rise to efferent fibres 

 which leave the pineal organ and pass to such ganglia as the habenular 

 or optic thalami, or to the plexuses of sympathetic nerve-fibres on the 

 surrounding blood-vessels. Moreover, one might look for two types of 

 nerve-cells, a large ganglion-cell belonging to the central nervous system 

 and a small type of nerve-cell having the characteristics of the sympathetic 

 system. 



Actually, it appears that if observations on the pineal system through- 

 out the whole series of vertebrate animals are included, all these different 

 types of sensory epithelial cells, nerve cells, and nerve-fibres have been 

 seen and described by competent observers. The pineal system, especially 

 that of the mammalia, is, however, vestigial in structure and has undergone 

 marked modifications, and as a consequence the full complement of 

 nerve cells and nerve-fibres is not found in any one species. Nerve cells, 

 in particular, are rare, and when present are usually not fully developed. 

 Such cells have been described as " neuronoid cells " or " amacrine 

 nerve-cells." Moreover, the existence of typical nerve cells showing both 

 Nissl granules and axis cylinder process as a normal constituent of the 

 human pineal organ has been not only doubted but denied by some 

 recent workers, who regard the occasional occurrence of such cells as 

 anomalous. 



There seem, however, to be transitional stages between nerve cells 

 and typical parenchyma cells, and it is probable that in some cases 

 branched pineal cells with bulbous extremities have been mistaken for 

 fully developed nerve cells. True nerve cells, apparently belonging to 

 the sympathetic system, are occasionally seen on or near the surface of 

 the organ or in close relation with the vessels contained in the trabecular, 

 and in our opinion a distinction should be made between these cells and 

 the transitional or " neuronoid cells " seen in the parenchyma. It is 

 possible that the latter indicate a stage in the differentiation of true nerve 

 cells from the indifferent neuro-epithelial cells which form the primary 

 elements of the developing organ, and which may give rise to neuroglial 

 cells, parenchyma cells, or very occasionally to either imperfectly or fully 

 developed nerve cells. 



The question of whether the parenchyma cells themselves are sensory 

 in nature and capable of transmitting a sensory impulse from an afferent 

 pineal nerve to an efferent pineal nerve is one of practical interest. Should 

 they possess this function, their anatomical connections fully warrant 

 the assumption that a reflex mechanism may exist within the pineal organ, 



