of the Fishery Board for Scotland. 



167 



sary to produce tetanus at far too high a number. Kronecker and 

 Stirling, in their investigations on the " Genesis of Tetanus," * showed 

 that in this Ranvier was in error, as they found that 10 stimuli per 

 second cause a tolerably complete tetanus of the red muscles, while the 

 pale muscles of the rabbit require 20-30 stimuli per second to produce 

 complete tetanus. The duration of the contraction of the red muscles 

 further is nearly twice as long as that of the pale. The red muscles, 

 therefore, contract more slowly, and their contraction is more sustained 

 and prolonged. They thus form a less highly evolved form of muscular 

 tissue — i.e., speaking physiologically as regards mere rapidity of contrac- 

 tion. 



Meyer has described similar differences in the colour and general struc- 

 ture of the muscles of the guinea pig. f 



A similar distinction exists also in the muscles of insects, as shown by 

 R. V. Limbeck. J Thus in the common water beetle, Dysticus niarginalis, 

 even with the unaided eye, on opening the abdomen and thorax, one sees a 

 sharp contrast in the colour of the muscles exposed to view. While the 

 muscles in the abdomen are pale, the muscles which almost fill the thorax 

 have a yellowish, indeed almost a brownish colour, and are thus sharply 

 marked off from the others. They differ markedly in structure. 



Fishes. — Gunther says, as regards fishes, that ' Each lateral muscle is 



* divided by a median lodgitudinal groove into a dorsal and ventral half ; 



* the depression in its middle is filled by an embryonal muscular sub- 



* stance, which contains a large quantity of fat and blood-vessels, and 



* therefore differs from ordinary muscle by its softer consistency, and by 



* its colour, which is reddish or grayish.' § 



Arrangement of Muscles. — The muscles of an osseous fish are arranged 

 into vertical segments or myotomes corresponding in number with the 

 vertebrae, and separated from one another by flat septa of connective 

 tissue, while the fibres run in a fore and aft or horizontal course from one 

 septum to the next one. They are therefore very short, and the con- 

 nective tissue is peculiar in this respect, that boiling for a few minutes 

 suffices to dissolve it, so that the muscular myotomes are easily separated 

 one from another. The myotomes are connected internally with the 

 bones of the vertebral column, and externally with the skin. The 

 myotomes do not run in the same vertical plane, but pursue a zig-zag 

 course. From the dorsal middle line they incline backwards then for- 

 wards, then backwards, and then bend at an angle forwards to reach the 

 middle line (Plate IV. fig. 1), so that the apices of the parts above 

 the middle line look backwards, and those at it look forwards, while 

 below this line another series of apices are directed towards the tail. 

 One set is described as a dorsal longitudinal muscle with tendinous 

 insertions II and the other as a ventral longitudinal muscle. 



Colour of the Muscles. — I daresay every one is more or less familiar 

 with the fact that, when a herring is boiled and the skin stripped off, a 

 thin layer of a reddish-brown tint is seen chiefly along the sides, and 

 shading off dorsally and ventrally under this is the paler muscle. In the 

 unboiled or fresh condition there is also a sharp difference in colour 

 between these two muscles, the darker one corresponds to the red muscles 

 of rabbits, and the deeper and far bulkier myotomes of lighter tint to the 



* Journal of Physiol, i., p. 384. 



t Uber rothe u. blase quergest. Muskeln, ArcJiiv f. Physiol, von du Bois Rey- 

 mond, p. 217. 1875. 



X Zvu" Kenntniss d. Banes d. Insectenmuskeln. Sitzb. d. k. Akad. d. Wis- 

 unsch. Math. Nat. Clas. Bd. xci. 1885. 



§ Introduction to the Study of Fishes, p. 93. 



li Owen, Com^. Anat. and Phys. of Vertebrates, i. p. 204. 



