NERVE SUPPLY OF MAMMALIAN PINEAL ORGAN 409 



on the surface of the brain between the cerebral hemispheres and cere- 

 bellum. Its only apparent functional connection with the organ is vascular, 

 and its nerve supply reaches it only in the form of non-medullated fibres 

 accompanying the blood-vessels (Cajal). 



The direction in which nerve impulses travel in the fibres connecting 

 the epiphysis with the habenular ganglia, optic thalami, and the superior 

 and posterior commissures is difficult to determine in mammalia, owing 

 to the absence of experimental evidence. It seems, however, to be 

 generally assumed that impulses travelling from the central fibres coming 

 from the posterior commissure through the " tractus intercalaris " not 

 only enter the stalk of the epiphysis, but are also distributed in the 

 parenchyma of the epiphysis. 



Pastori states that in some species of mammals (e.g. man and dog) 

 the nerve-fibres coming from the optic thalami and habenular ganglia 

 partly decussate in the inter-habenular commissure ; while in other species 

 of mammals (e.g. the cat) the corresponding nerve-fibres remain homo- 

 lateral and travel directly from the optic thalamus and habenular ganglion 

 into the parenchyma of the epiphysis, that is to say, without decussating. 



On the other hand, the primary direction in which the nerve impulses 

 travel in the lower classes of vertebrates is apparently from the sensory 

 cells of the pineal eye, pineal sac, or epiphysis to the central ganglia — 

 vide Beraneck, Dendy, Gaskell, Klinckowstrom, Studnicka, and others. 

 Moreover, some writers have supposed that the parenchyma cells of the 

 human epiphysis may be sensory, or receptor, cells ; and it has also been 

 suggested that they may be specially sensitive to pressure, and, further, 

 that they may function in regulating the pressure of the cerebrospinal 

 fluid, either through the direct action of the sympathetic system on the 

 choroidal epithelium or by an indirect action on the epithelium through 

 the choroidal blood-vessels. 



These considerations suggest that relays of nerve-fibres which originally 

 carried impulses from the receptive organs of the pineal system to ganglia 

 of the central nervous system have been either wholly or partially sup- 

 planted by nerves which are afferent to the epiphysis, and also that 

 impulses arising by stimulation of the parenchyma cells of the epiphysis 

 may be transferred to fibres of the sympathetic system. 



An anatomical basis which affords support for the latter hypothesis 

 is furnished by Pastori's recent work on the nervous connections of the 

 epiphysis. He has demonstrated in the human subject and in the dog 

 the constant presence of a sympathetic ganglion situated in the mem- 

 branes just behind the posterior pole of the epiphysis. This ganglion 

 is connected by a large number of very fine nerve-fibres with the epiphysis, 

 and also by less numerous but coarser nerve-fibres, which form a definite 



