WILLIAMS : PHENOMENA OF MUSCLE- CONTRACTION. 49 



■||- sec. in Scaphander Hgfiarms to ^^^^ sec. in Sepia officinalis. 

 But while, as is evident from their structure, the kind of rnuscle 

 in the vertebrates that they most resemble is the unstriated, yet 

 they differ very greatly from that variety in being, as Varigny 

 has demonstrated, ^ under the control of the will, sometimes 

 very rapid in their contraction and relaxation, and, taking them 

 on the whole, not less irritable than striated muscle. The only 

 one analogy that comes to my mind as existing in the verte- 

 brates — and I think it is the only one that can be safely 

 assumed — is the ciliary muscle of the iris, which, though con- 

 sisting of unstriped fibres, is yet under voluntary control through 

 the medium of the third cranial nerve. But the work they can 

 perform is great. Simroth ^ states that a small Helix can still 

 move along when burdened with a weight nine times its own, 

 and E. Sandford * has proved that a H. aspersa weighing \ oz. 

 can draw along a horizontal plane a weight weighing 17 ozs., 

 which is still much greater (51 times its own weight) and that 

 another of the same species with a weight of \ oz. can drag up a 

 vertical plane 2\ oz. (nine times its own). But to have estimates 

 a little more absolute. The absolute force of one square centi- 

 metre of the muscle of a frog is from 2 '8 to 3 kilogrammeters 

 and about 8 or 9 kilogrammeters in man. The mollusca do not 

 fall short of these estimates. Plateau ^ has found that in the 

 adductor muscles oi Pectimcuhis glycynieris it is equal to 10,152 

 grammes; in Fecten maximus, 3,786; in Mytilus ediilis, 7,984; 

 in Ostrea edidis, 5,867; in Venus verrucosa^ 12,431; in Mya 

 arenaria, 1,178; in Cardiuni edule, 2,856; in Fecten opercu- 

 laris, 530; and in Tridacna, 1,595 grammes. It should, how- 

 ever, be mentioned that the adductor muscles of the genus 



^ "Sur quelque points de la physiologic des muscles lisses chez les 

 invertebres." Op. cit., ci, pp. 656 — 658. 



■■^ "Die Thatigkeit der willkiirlichen Muskulatur unserer Landsch- 

 necken." Z. Wiss, Zool. xxx suppl. voL pp. 166 — 224. 



* "Experiments to test the strength of snails (Helix aspersa)." Zoo- 

 logist (3) X, p. 491. 



^ "Recherches sur la force absolue des muscles des invertebres." i 

 Partie, Bull. Ac. Belg. (3) vi, pp. 226—259. 



