190 THE PINEAL ORGAN 



is usually situated behind and below the cavity of the main vesicle with 

 which it communicates in the majority of cases, although it is in some 

 completely cut off and appears as an independent closed vesicle lying 

 between the proximal end of the funnel-shaped cavity and the exit of the 

 pineal nerve. The wall of the atrium is composed of cells which are 

 columnar in type. The free ends of these converge towards the central 

 cavity, while their nuclei are peripheral (Fig. 45). Around these cells 

 is an accumulation of irregularly disposed cells, the nuclei of which 

 resemble those of the surrounding ganglion cells. The atrium therefore 

 has been thought by some writers to represent a nerve ganglion situated 

 in the wall of the vesicle and interposed between the sensory or visual cells 

 and the tract of nerve-fibres which connect it with the habenular ganglion. 

 Others, however, considering the similarity of the columnar cells lining 

 its cavity to the columnar cells of the retina, regard the atrium as a part 

 of the general cavity of the organ which has been either wholly or partially 

 constricted off, and corresponds either to a secondary diverticulum 

 from the main outgrowth or a part of the cavity of its stalk. According 

 to Kuppfer (1894) tne primary outgrowth of the diverticulum from the 

 roof of the interbrain is at first directed backward ; later an extension 

 forward takes place, so that in a median sagittal section its cavity including 

 that of the stalk appears T-shaped, and it seems possible that the posterior 

 limb of the cross-bar of the T persists as the atrium whereas the distal 

 part of the vertical bar remains as the stem of the funnel. 



The retina (Figs. 45 and 46, Chap. 3, pp. 69, 70) consists of (1) sensory 

 cells ; (2) tall, columnar cells containing the so-called " white pigment " ; 

 (3) ganglion-cells lying in a plexiform layer of nerve-fibres, with a few 

 spindle-shaped neuroglial or connective tissue cells. There are also an 

 internal and an external limiting membrane and a thin fibrous capsule 

 continuous with the pia mater of the brain. The sensory or visual cells 

 end distally in a flask-shaped or bulbar swelling covered over by a cap of 

 clear finely granular material which frequently tapers at its extremity 

 into a fine thread which is continuous with a syncytial fibre of the vitreous. 

 The base of the knob or flask rests on the internal limiting membrane and 

 is continuous through the membrane with the thin rod-like outer end of 

 the body of the cell ; the oval nucleus is situated in a spindle-shaped 

 swelling of the body near the base of the cell. Proximal to the nucleus, 

 the body of the cell tapers into a fine nerve process which subdivides in a 

 plexus of nerve-fibres containing the ganglion cells, the axons of which 

 course in a tangential direction in the latter and join to form the pineal nerve. 



The interpretation of specimens prepared by the silver impregnation 

 methods is difficult. Cells which are heavily impregnated are found in 

 the lens, and according to Studnicka's description of specimens prepared 



