ANIMAL MECHANICS. 



165 



II. Muscular Fibres forming Curved Surfaces. 



i . Fibres rectilinear, 

 (a) Skew Surfaces. 



2. Fibres curvilinear, 

 (a) Ellipsoidal Surfaces. 



Fibres in same plane, parallel to each other, and rectilinear. 

 — This natural group of muscles has been described by Bo- 

 relli,* who has named their several species, Direct Prismatic, 

 Bhomboidal, and Pemiiform. 



Direct Prismatic Muscles. — If we suppose two bars, A A 

 and BB, joined by a number of parallel elastic springs, to be 

 drawn asunder, as re- ^- — cT~ y " 

 presented in Fig. 22, 

 the elasticity of the [ 

 strings will draw back A 

 "the bars into their origi- 

 nal position, by the same 

 kind of action as the con- 

 tractile fibres of a Direct 

 Prismatic muscle. If f 

 denote the contractile 

 force of each fibre, and 

 n their number, the re- 

 sultant force of the mus- 

 cle will be 



• /''=«/. (23) 



and will act parallel to 

 the fibres, in the di- 

 rection ab. If the pa- 

 rallel sheets of mus- 

 cular fibres be united, so as to form a solid muscle, the re 

 * Do Motu Animalium, vol. L, p. 8. 



