THE ELASMOBRANCH FISHES 165 



pliysis (»f the Iowcm- jaw aiul giving l)ranches to the coracomandibularis mus- 

 cle; and the other upward between the hyoid and mandibular cartilages to 

 supply the structures around the mandibular and hyoid regions. The external 

 carotid or one of its branches gives off a small branch (th.\ fig. 153) which 

 supplies the anterior part of the thyroid gland. 



From the upper third of the first efferent-collector another large artery, the 

 pseudobranchial (jjs., figs. 151 and 152) , runs forward to break up into a num- 

 ber of strong branches in the spiracle. From the spiracle this artery is con- 

 tinued inward and forward as the ramus anastomoticus {r.a., fig. 152), which 

 passes through a foramen in the orbit (see fig. 47, f.r.a., facing p. 44) to join 

 the internal carotid artery inside the cranial wall. Before entering the orbit, 

 however, this ramus gives off the ophthalmica magna (o.m., fig. 152) to the 



eye region. 



EFFEEENT AETERIES 



The first efferent artery, the hyoidean efferent (hy.ef., fig. 152), extends as a 

 continuation of the first efferent-collector from the anterodorsal angle of the 

 first efferent-collector loop forward and mediad to the paired dorsal aorta 

 {d.a.'). The remaining efferents, branchial efferents {br.ef.^-'^, fig. 151) simi- 

 larly arise from the dorsal angles of efferent-collector loops, but they, as con- 

 tinuations of the anterior efferent-collector, extend backward and inward to 

 join the unpaired dorsal aorta. In Heptanchus five of these efferents, above 

 the first to the fifth holobranchs, reach the unpaired dorsal aorta and a sixth 

 joins the fifth. 



Near the union of the hyoidean efferent and the paired dorsal aorta an or- 

 bital (stapedial) artery {or., fig. 152) is given off laterally and extends for- 

 ward through the orbit. The paired dorsal aorta is continued forward as the 

 internal carotid (i.e., fig. 152) which enters the cranium from the midventral 

 line to supply the brain. After perforating tlie cartilage the right internal 

 carotid joins the left and the two run for a short distance as a common trunk. 

 They then separate and pass forward and slightly outward where they are 

 joined by the right and the left ramus anastomoticus, respectively (r.a., fig. 

 152) . Given off after the union of the ramus anastomoticus and internal caro- 

 tid are the three cerebral arteries to the brain. The anterior cerebral (a.c, 

 fig. 152) passes anteriorly between the hemispheres of the brain and is usually 

 joined to its fellow from the opposite side by an intercommunicating artery. 

 The median cerebral (m.c.) passes forward to supply the olfactory tract and 

 bulb. The posterior cerebral (p.c.) passes backward, and its two branches join 

 to form the arteria spinalis (a.sp., fig. 152) which extends down and ventral 

 to the spinal cord. There is thus formed by the union of the anterior and the 

 posterior cerebrals a complete circle around the ventral part of the brain. 



Dorsal Aorta 



The dorsal aorta (fig. 152) is composed of a short anterior paired part (d.a.^) 

 and a long posterior unpaired part (d.a.) . The paired part receives the hy- 

 oidean efferent, and the unpaired part receives the first four pairs of efferents 



