Occurrence of a Mesoccelic Recess in the Human Brain. 515 



Fig. 1. — Transverse section through the abdominal chromaphil body of the dog. Fixed 



in corrosive sublimate and stained with hematoxylin. Section 10 /i in thickness. 



Leitz, obj. 6. Drawing oculai". 

 Fig. 2. — Section through the medulla of the adrenal of a dog, prepared as in case of 



previous figure. Same magnification. 

 Fig. 3. — Section through a group of chromaphil cells in the inferior cervical ganglion of 



the dog. Fixed in Kohn's fiuid (bichromate-formol, see text). Leitz, oil 



immersion. Drawing ocular. 

 Fig. 4. — Transverse section through the abdominal chromaphil body of the dog. Fixed 



in Kohn's fluid (v. supra), stained with htematoxylin. Leitz, y^" oil immersion. 



Drawing ocular. 



Fig. 5. — Section through the medulla of the adrenal of a dog. Kohn's fluid. Hrema- 

 toxylin. Leitz, obj. 6. Drawing ocular. 



On the Occurrence of a Mesoccelic Recess in the Human Brain, 

 and its Relation to the Suh- Commissural Organ of Lower 

 Vertebrates ; tvith special 7'eference to the Distribution of 

 Reissners Fibre in the Vertebrate Series and its possible 

 Function. 



By Arthur Dendy, D.Sc, F.E.S., Professor of Zoology in King's College 

 (University of London), and G. E. NiCHOLLS, B.Sc, Assistant-Lecturer 

 and Demonstrator in Zoology in King's College. 



(Received May 24,— Read June 2, 1910.) 



[Plate 14.] 



I. Introductory. 



Eight years ago one of us (Dendy, 1902) published in the ' Proceedings ' of 

 the Royal Society a description of a peculiar structure lying jbeneath the 

 posterior commissure in the brain of the Ammocoete, in the shape of a pair of 

 longitudinal grooves lined by greatly elongated and apparently ciliated 

 columnar cells, and evidently formed by specialisation of the ependymal 

 epithelium which lines the cavity of the brain. Previous to this time there 

 appear to have been, at the most, only a few scattered references to the 

 occurrence of any such structure in the vertebrate brain. We now know, 

 however, that it occurs throughout the whole vertebrate series, from the 

 lampreys to the primates, it having recently been figured, in the case of 

 Macacus, by Sir Victor Horsley (1908). 



VOL. LXXXII. — B. 2 R 



