Royal Society. 279 



The author concludes by pointing out the more immediate affini- 

 ties of tlie I3elemniti's, and showing that it combines characteristics 

 which are now divided amongst distinct genera: as, for example, 

 first, a com])lex internal shell, divisible into the same principal parts 

 as that of the Sepia, but one of whicii has, secondly, the same essen- 

 tial chambered structure as the shell of tiie Spinila ; thirdly, unci- 

 nated cephalic arms, as in the Ouijcltotentliis \ and lastly, an ad- 

 vanced position of rounded fins, as in the Spirula and Rossia. 



The paper is illustrated by drawings of the specimens described, 

 ■with microscopic views of the shell and nmscular tissue, and a re- 

 storation of the Belemnite according to the data aftbrded by the pre- 

 sent fossils. 



June 20 "On the Structure of the Ultimate Fibril of the Muscle 



of Animal Life." By Erasmus Wilson, Esq., Lecturer on Anatomy 

 and Physiology in the Middlesex Hospital ; in a Letter addressed to 

 Peter Mark Roget, M.D., Sec. R.S. Communicated by Dr. Roget. 



By resorting to peculiar methods of manipulation, and employing 

 a microscope of more than ordinary power, the author, -with the 

 assistance of Mr. Lealand, has succeeded in discovering the real 

 structure of the ultimate muscular fibril, in a specimen taken from 

 the arm of a strong healthy man immediately after its amputation. 

 He finds each fibril to be composed of minute cells, disposed in a 

 linear series, flattened at their surfaces of apposition, and so com- 

 pressed in the longitudinal direction as to leave no marginal in- 

 dentation on the surface ; thus constituting a uniform cylinder, 

 divided into minute subdivisions by transverse septa, which are 

 formed by the adherent surfaces of contiguous cells. The diameter 

 of the fibril, in the state of relaxation, is the 20,000th part of an 

 inch. The cells are filled with a transparent substance, to which 

 the author gives the name oi 31yoline, and which diff"ers in its re- 

 fractive density in different cells. In four consecutive cells the 

 myoline is of greater density than in the four succeeding cells, and 

 this alternation is repeated throughout the whole course of the fibril. 

 In consequence of all the fibrils composing the ultimate fasciculus 

 having the same structure, and the cells, which are in lateral juxta- 

 position, containing myoline of the same density, they act similarly 

 on light, and the whole presents, to the eye of the microscopic ob- 

 server, a succession of striae or bands, dark and luminous alternately, 

 and transverse to the direction of the fasciculus ; an appearance 

 which has been noticed by preceding observers, but of which the 

 cause had not hitherto been ascertained. A dark stria may occa- 

 sionally appear as a luminous one, and vice versa, when viewed by 

 light transmitted at different degrees of obliquity. 



The structure here described, the author remarks, reduces the 

 muscular fibre to the simple type of organization exhibited in the 

 combination of a series of cells, associating it with other tissues of 

 cell formation, and will probably, he thinks, open new sources of 

 explanation of the immediate agency of muscular action, a power 

 hithertoMnvolved in the deepest mystery. 



" On the Reproduction of lost parts in Myriapoda and Insecta." 



