12 ZOOTOMY. 



arranged in transverse segments, called myotomes or 

 myocommas, separated from one another by strong plates 

 of fibrous tissue. 



Each myotome takes a zigzag course ; starting from the middle dorsal 

 line, it passes first sharply backwards, then more gently forwards, then 

 backwards, and finally sharply forwards. The plane of the myotomes 

 is not at right angles to the long axis of the body, but is inclined from 

 the surface, inwards and forwards. The muscular fibres of which they 

 are composed are longitudinal, i.e. at right angles to the general direction 

 of the myotomes themselves. 



d.ao 



FIG. 6. Petromyzon marinus. Transverse section through the 

 branchial region, semi-diagrammatic (nat. size). The gill sacs are 

 supposed to be rotated forwards, so as to bring the external in the 

 same plane as the internal branchial apertures. 



br.m., branchial membrane : d.ao, dorsal aorta : d.c, dorsal carti- 

 lage of branchial basket : d.m, dorsal body-muscles : e.a, external 

 branchial aperture : f.t, fibrous tissue inclosing neural canal : //, i, 

 lateral longitudinal cartilages of branchial basket : i.a, internal branchial 

 aperture : t.j'u, inferior jugular vein : /, jugular vein : my, myelon : 

 nc, notochord : n.ca, neural canal : n.p, neural processes : as, oeso- 

 phagus : p.br, peri-branchial sinus : r.m.t, retractor muscle of tongue : 

 r.t, respiratory tube : s, circum-resopha.^eal sinus : v.ao, ventral aorta ; 

 v.c, ventral cartilage of branchial basket : v.m, ventral body-muscle. 



34. The division of the myotomes, immediately behind 

 the last gill-cleft, into two longitudinal masses, a dorsal and 



