MINUTE STRUCTURE. 



563 



translucent material is seen to be divided up by connective tissue septa, 

 but there is no obvious arrangement of columns. 



Minute structure.— The most important features of the minute 

 structure of the organs just described are those of the columnar 

 constituents. 



Every column bounded by connective tissue walls, is itself sub- 

 divided at regular intervals by connective tissue cross septa, which 

 are fixed at their peripheral edges to the boundary walls ; the column 

 thus consists of a pile of superimposed compartments. Further, each 

 of these compartments contains a special protoplasmic mass in which 

 efferent nerves end, whilst the remaining portion of the space offered by 

 the compartment is filled by a transparent material. The protoplasmic 

 mass, with its supporting fibres and nerve terminations, constitutes 

 what is termed the electric plate or disc, and is well supplied with 

 blood vessels. 



The structure of the plates differs as regards minute detail in the 

 various groups of fish just mentioned, although all present general 

 features of resem- 

 blance. It is best 

 understood by an ex- 

 amination of the least 

 complicated organ, 

 that of the skate. 



In the organ of the 

 skate, the columns of 

 which it is composed 

 are divided into the 

 compartments pre- 

 viously referred to ; 

 along the connective 

 tissue walls of these 

 compartments run 

 the blood vessels and 

 nerves which enter to 

 be distributed in the 

 plate (see Fig. 291). 

 The plate or disc is slung across the compartment, being attached by its 

 peripheral edges to the lateral walls, and in contact by its free surfaces 

 with the semi-fluid albuminous material which fills the remainder of 

 the compartment, as shown in Fig. 292. x The cephalic border receives a 

 dense network of nerve fibres, the dichotomous divisions of which enter 

 along this surface. Beneath this the nerve branches appear to end in 

 groups of twigs, with terminal knobs lying in a multinucleated layer of 

 granular protoplasm. Immediately beneath this in the protoplasmic 

 layer is a remarkable band of wavy fibres ; these are all parallel, and 

 run across the plate between the two lateral attachments, in such a 

 position as to form the most central portion of the disc. These mi- 

 branched fibres are quite distinct from one another, and are very 

 prominent in preparations which have been fixed in osmic acid. This 



1 For structure of organ of skate, see Stark, Proc. Roy. Soc. Edin., 1844-45, vol. ii. ; 

 Sanderson and Gotch, Journ. Physiol., Cambridge and London, 188S, vol. ix. ; Ewart, 

 Phil. Trans., London, 1888, vol. clxxix. ; 1892, vol. clxxxiii. ; Engelmann, Arch. f. d. ges. 

 Physiol., Bonn, 1894, Bd. lvii. ; G. Retzius, Biol. Untersxbch., Stockholm, 1898, Bd. viii. 

 S. 83. 



Fig. 291. — Electrical organ of skate. Longitudinal section of 

 upper part of organ, as seen under a low power. The 

 arrow indicates the direction of the shock. 



