May 25, 1893] 



NATURE 



The action upon the circulatory, muscular and nervous systems, 

 therefore, closely resembles of that strophanlhin. 



April 27. — "The Electric Organ of the Skate. Note 

 on an Electric Centre in the Spinal Cord." By J. C. 

 Ewart, M.D., Regius Professor of Natural History, Univer- 

 sity of Edinburgh. Communicated by Prof. Sir W. Turner, 

 F.R.S. 



Having considered the development and structure of the 

 electric organ of the Skate, it appeared to me desirable, by way 

 of making my work more complete, to reinvestigate the nervous 

 apparatus of the organ, and more especially to ascertain whether, 

 as in Torpedo and Gymnotus, there is an electric centre. In 

 Torpedo the electric organs are developed from a limited number 

 of myotomes, and innervated by afferent fibres, belonging to a 

 limited number of cranial nerves, which proceed from two large 

 collections of cells — the electric lobes— situated in the region of 

 the medulla. In Gymnolus the nerves for the electric organs 

 proceed from two well-marked cellular tracts which extend 

 along the greater length of the spinal cord, one at each side of 

 the central canal. In the case of the Skate the 

 question at the outset is, granting the existence of 

 an electric centre, is it, as in Torpedo, situated in 

 the brain or, :1s in Gymnotus, in the spinal cord ? 

 Sanderson and Gotch [Journal of Physiology, 

 vol. X. No. 4), made out that in the Skate "a 

 reflex centre is situated in the optic lobes," but, 

 notwithstanding this, these lobes in the Skate in 

 no way differ histologically from the corresponding 

 structures in AcaiUhias and other Selachians 

 unprovided with electrical organs. 



The development of the Skate's organ from 

 portions of the caudal myotomes, and its inner- 

 vation by afferent fibres from certain caudal nerves, 

 point to the electric centre being situated in the 

 spinal cord rather than in the brain, and to its 

 being, as in Gymnotus, on a level, and all but 

 coextensive, with the electric organ. 



Having observed, when working at the develop- 

 ment of the electric organ, a number of large 

 nerve-cells in the caudal portion of the spinal cord, 

 the sections of Skate embryos made some years ago 

 were first examined. It soon became evident that 

 in sections from the middle of the tail on a level 

 with the electric organ certain cells of the anterior 

 horn of the cord were very much larger than in 

 sections through the root of the tail, and further 

 that in late embryos and very young Skate there 

 was an electric centre, resembling in many respects 

 the electric centre in Gymnotus. 



It did not, of course, follow that the electric 

 nerve-cells persisted into adult life. They might 

 degenerate, and thus the supposed feebleness of 

 the Skate's organ might be accounted for. The 

 fact that the Skate's organ increases in size as the 

 fish grows larger led me, however, to expect that 

 large nerve-cells would be found in the caudal 

 region of the spinal cord in well-grown fish. In 

 this I was not disappointed, for, though there was 

 at first some difiiculty in demonstrating the presence 

 of electric nerve-cells in large fish, on obtaining 

 perfectly fresh material their position, size, and 

 relations were easily made out, and the remark- 

 able difference in the appearance of sections of 

 the cord at, and in front of, the root of the tail, 

 from sections on a level with the electric organ, was at once 

 evident. From the observations already made, it appears that 

 the electric centre in the Skate closely resembles, from a 

 morphological point of view at least, the electric centre in 

 Gymnotus. The electric tract is, however, much shorter in 

 the Skate than in the Electric Eel, and the cells are relatively 

 fewer in number. On the other hand, the cells in the Skate 

 are larger than in Gymnotus, and this is true not only of 

 Raia balis but also of R. radiata, in which the organ is ex- 

 tremely small and poorly developed. Nerve-cells from the 

 electric centres of Torpedo, Gymnotus, and Raia are repre- 

 sented in the accompanying figures. Fig. i represents a 

 cell from the electric centre of the Skate (a R. talis two feet 

 in length) ; Fig. 2 a cell from the electric centre of a well- 

 grown Gymnotus : and Fig. 3 a cell from the electric lobe of 

 a large Torpedo. All three figures are camera drawings, and 



NO. 1230, VOL. 48] 



the same lenses were used in each case — objective D and ocular 

 2, Zeiss. It will be noted that, though the cell from the Skate 

 is much smaller than the Torpedo cell, it is decidedly larger than 

 the one from Gymnotus. 



In sections of the Skate's cord on a level with the electric 

 organ, small, as well as large, cells are usually visible in the 

 anterior horn. The small cells are in connection with the 

 fibres which supply the untransformed caudal muscles. They 

 agree exactly with the cells in the anterior horn throughout the 

 entire length of the spinal cord lying in front of the electric 

 organ region. One of these unenlarged motor cells is repre- 

 sented in Fig. 4. It was drawn from a section of the cord (of 

 the same fish from which Fig. I was taken), about six inches in 

 front of the electric organ. It closely resembles, except in size, 

 the electric cell (Fig. i), and it also resembles the large motor 

 cells of the Mammalian cord. A motor cell from the spinal 

 cord of a Mammal, drawn to the same scale as the other cells 

 given, is represented in Fig. 5.^ This cell, smaller than the 

 electric cell of the Skate (i), and still smaller than the cell from 



C.C. 



Torpedo (3), is about the same size as the electric cell of 

 Gyvmotus (2). 



With the help of sections through a series of embryo Skate, 

 for most of which I was indebted to Dr. Beard, I have been 

 able to study the development of the cells in the Skate's electric 

 centre. This part of the sul.ject, together with the condition of 

 the electric cells in large fish, will be dealt with in a subsequent 

 communication. It may, however, be stated now : (l) That in 

 R. batis embryos under 5 cm. in length none ol the motor cells 

 in the caudal region had undergone enlargement. (2) That in 

 an embryo 5 '8 cm. in length, although the muscular fibres 

 seemed still unchanged, certain cells in the anterior horn of the 

 caudal portion of the cord were disii tcily larger than similarly- 

 shaped cells in their vicinity. (3) That in an embryo I5'5 cm. 



For tlie use of the section from which I i^. 5 was drawn I am indebtei 

 to Sir William 'riirner, F.R.S. 



