THE ELASMOBRANCII FISHES 



91 



The second dorsal constrictor (csd.-) is a very broad l)and lying between the 

 spiracle and the first branchial cleft. It arises also along the fascia of the 

 dorsal bundle, its anterior margin in Heptanchus maculatns being continu- 

 ous with the first dorsal constrictor. In Heptanchus cinereus it is overlapped 

 by the first constrictor. From its place of origin it extends downward and its 

 anterior and more superficial fibers are inserted on the quadrate; the fibers 

 lying underneath the superficial fibers join the upper segment of the hyoid; 

 while those fibers lying back of them pass over to join the fibers of the ventral 

 constrictor. In Heptanchus maculatns, where the dorsal constrictor meets the 

 ventral there is superficially a heavy connective tissue septum. 



CSV. 



CSV. 



Fig. 92. Lateral view of the constrictor muscles, Heptanchus mactdatu.s. (From Davidson.) 



a.rnd., adductor mandibulae ; a.pr., antorbital process; csd.^-^, first to eighth dorsal con- 

 strictors; csv.^'^, second to eighth ventral constrictors; Us., medial levator labialis; lls.^, 

 lateral levator labialis; l.mx., levator maxillae; md., mandible; po.o., postorbital process; 

 p-q., palatoquadrate (pterygoquadrate) ; sc, scapular cartilage; sp., spiracular cleft; tr., 

 trapezius muscle. 



The third to the eighth {csd.^) dorsal constrictors are more slender and have 

 their origin largely from the dorsal fascia and by tendons through the tra- 

 pezius muscle {tr.). Tlie superficial fibers pass over into those of the ventral 

 constrictors, while deeper fibers, acting as interbranchial muscles (see p. 149, 

 fig. 143, ib.cl.), lie just anterior to the cartilaginous branchial rays. In a section 

 cutting through the dorsal constrictor between two gill clefts, the dorsal con- 

 strictors (csd.) are the thicker bundles lying over the margin of the septum; 

 and the deeper fibers (ib.d.), comparable to the interbranchial muscle of the 

 more specialized Elasmobranehs, extend inward as thinner bands. 



VENTRAL CONSTEICTORS 



The ventral constrictors of Heptanchus cinereus are seen in figure 93. They 

 have their origin from a seam of connective tissue in the midventral line. The 

 superficial, posterior fibers pass over into the dorsal constrictors, but the 

 deeper ones are inserted on the ceratobranchial segments of the visceral arches. 



