90 



THE ELASMOBRANCH FISHES 



of the four rectus muscles, all of which arise from the posterior surface of the 

 orbit around the base of the optic pedicel (o.p.) . The most dorsal of the rectus 

 muscles is the superior rectus (s.r.), the most ventral the inferior rectus {i.r.), 

 the most posterior the external or posterior rectus (p.r.), and the most ante- 

 rior the internal or anterior rectus (a.r.). They pass outward and forward, 

 also to be inserted on the eyeball. 



Buccal and Pharyngeal Muscles 



The muscles in the region of the mouth and pharynx may be separated into 



four groups. The first group includes the superficial constrictor muscles; the 



second comprises the interarcuales ; the third 

 the adductors; and the fourth the hypobran- 

 chial muscles. 



The superficial constrictor muscles are vis- 

 ible upon removal of the skin from the re- 

 gion around the gill clefts. The levator maxil- 

 lae il.mx., fig. 92) apparently is the forward 

 continuation of these muscles. It lies just 

 back of the eye and has its origin on the cra- 

 nium under the postorbital process {po.o.) ; 

 it extends from its origin downward and for- 

 ward to the palatoquadrate cartilage (p-q.). 

 The constrictors appear as eight dorsoven- 

 tral bands (csd.'^'^) separating the gill clefts 

 and functioning in their closure. Each con- 

 strictor, except the first, may be considered 

 to be made up of dorsal and ventral com- 

 ponents, although the two in Hepfanch us are 

 more or less continuous. The constrictors 

 l)ound tlie clefts so that the first and second 

 are separated by the spiracular cleft (sp.) 



above, and the 2-3, 3-4, etc., are separated by the cleft following. The last of 



these superficial muscles lies in front of the last cleft. 



Fig. 91. Muscles of the eye, Hcp- 

 lanchus maculatus, dorsal view. 



a.r., anterior or internal rectus ; 

 i.o., inferior oblique muscle ; i.r., in- 

 ferior rectus ; n.II, optic nerve ; o.p., 

 optic pedicel; p.r., external or pos- 

 terior rectus ; s.o., superior oblique ; 

 s.r., superior rectus. 



DORSAL CONSTEICTORS 



The first dorsal constrictor (c.sd.'^) arises from the occipital part of the cra- 

 nium and from heavy connective tissue (fascia) surrounding the dorsal longi- 

 tudinal bundles. As a thin slip of muscle it passes downward and slightly 

 backward around the anterior border of the spiracle to be inserted on the in- 

 ner and upper margin of the palatoquadrate cartilage (p-q.). Dorsally and 

 anteriorly the first dorsal constrictor is difficult to separate from the levator 

 maxillae muscle; furthermore the two have a common line of origin and of 

 insertion. 



