l888.] NEW-YORK MICROSCOPICAL SOCIETY. 29 



e., becoming shorter and thicker. In all vertebrates, but espe- 

 cially of the higher orders, they constitute the mass that is 

 called flesh. They are covered by tendinous sheaths, and are 

 united by the so-called connective tissue. But in insects the 

 fibres are not so united. They are deficient in the tendinous 

 sheaths, and are attached directly to the cutaneous exo-skeleton, 

 as in the crustacean. 



These muscle-fibres are divided into two kinds — striated and 

 smooth. The striated fibres are thus named because they are 

 apparently streaked at right angles across the longitudinal direc- 

 tion of the fibre. And they may be compared, as viewed micro- 

 scopically, to a roll of coin. It is a grave and unsettled ques- 

 tion as to what these striations or streaks are. When fibres are 

 treated with certain reagents, they separate along the lines of 

 the striations into discs. Other reagents, on the contrary, cause 

 them to split up longitudinally into still finer fibres. Dr. Rosen- 

 thal, in his work on " Muscles and Nerves," says: " It is impos- 

 sible to affirm that either the discoid or fibrinoid structures 

 actually exist in the muscle-fibre itself," and *'it can be shown 

 that the fibre, when taken from the living animal, is of a semi- 

 fluid consistency, and it must rather be assumed that both forms 

 of structure are really the results of the application of the re- 

 agents, that solidify the semi-fluid mass, and split it up in a 

 longitudinal or transverse direction." 



These striated muscle-fibres, in all animal life, are acted upon 

 directly through the nervous system by the will, and for this 

 reason are called the voluntary muscles. The smooth muscle- 

 fibres, on the contrary, are not so acted upon, and are termed 

 the involuntary muscles. There is but one exception to this 

 rule. The heart is provided with striated muscle-fibres, and 

 yet works independently of the will. Smooth muscle-fibres con- 

 tract by local irritation, produced by certain matter in the or- 

 gans which they surround, or of which they form a component 

 part. 



Wagner states that the muscular system of insects consists of 

 distinct, isolated, straight fibres, which are not gathered into 

 bundles, nor united by common tendons, and are often striated. 

 Siebold, on the contrary, states that not only the voluntary 

 muscles are striated, but often those of the organic life — /. e., 

 the involuntary muscles of the digestive organs — are striated. 



