48 COMPARATIVE ANATOMY chap. 



The above differentiations of muscle tissue are seen within the 

 division of the Cnidaria. Various forms of muscle elements can here 

 be found in one and the same animal. 



Even if it is not impossible that muscle fibres respond to certain 

 direct stimuli by contraction, it is generally characteristic of them 

 that they respond only to stimuli communicated to them by the nerve 

 elements. 



A general division into two principal types of muscle fibres may 

 be made ; (1) smooth, and (2) transversely striated. 



Smooth muscle fibres are almost always simple muscle cells. 

 One single cell forms a fibre. The contractile substance is either 

 formed by the cell on one side, so that the cell with the nucleus appear 

 as an attached body (Fig. 47, c, Cj), or the contractile substance becomes 

 differentiated on the whole surface of the formative cell, and then 

 tubular muscle fibres arise (Fig. 47, e, e^). In these we distinguish the 

 outer contractile eortiele layer from the central medulla which con- 

 tains the nucleus ; this medulla fills the axis of the muscle fibre, and 

 represents the more or less unchanged remnant of the protopilasm of 

 the formative cell. The contractile substance of the smooth muscle 

 fibres often appears longitudinally striated, and under the influence of 

 suitable reagents separates into the fine long fibrillae which are the 

 cause of this longitudinal striation. The smooth muscle fibres are 

 often branched at one or both ends, especially in those cases in which 

 they have an isolated course (Fig. 47, d, /). The above described muscle 

 fibres of the Ccelenterata are of this sort. 



Striated muscles. — These are considered as physiologically the 

 most efficient muscle elements. Even among the Codenterata, the con- 

 tractile fibres of the epithelial muscular cells appear transversely 

 striated. The muscle processes of several cells unite to form a group 

 of striated fibres. Striated fibres form the chief mass of the muscula- 

 ture in the Arthrojjoda and Vertebrata. They arise out of muscle- 

 forming cells, the greater portion of one such cell becoming 

 differentiated into transversely striated fibre. The remainder of the 

 protoplasm, with the nucleus, often persists on the surface of the fibre. 

 Perhaps it is this protoplasmic layer which produces the Sarcolemma, 

 the membrane which envelops the muscle fibre. In nearly all cases, 

 however, the number of nuclei increases as the muscle fibre grows and 

 differentiates further ; the fibre is thus to be considered as a many- 

 celled structure proceeding from one cell by incomplete division. 

 Often, however, several cells lying one behind the other share in 

 the formation of a muscle fibre. Striated muscle fibre appears not 

 only transversely but longitudinally striated. The transverse striation 

 comes from a regular alternation of singly refractive -with doubly 

 refractive elements. 



There are, at the present time, many different opinions about the finer structure 

 of the striated muscle fibre. According to the last view of Van Gehuchten, sup- 

 ported by investigations of the transversely striated muscle fibres in the Arthropoda, 



