THE ELASMOBRANCH FISHES 



183 



tliroui>li a foramen or pair of foramina in or near the middle line. Witliiii the 

 craninm it may be joined to the eorres|ionding internal carotid of the opposite 

 side by a cross-connective, as in S(/u<ifina; or the two may run for a short dis- 

 tance as a fused connnon trunk. The internal carotid then passes forward, and 

 after receiving the ranuis anastomoticus, gives off the optic artery (op., fig. 



Fig. 172. Cerebral arteries. A. Sqnalns sticMn. (E. H. Barbera, orig.) B. Cetorhinun. 

 (From Carazzi.) C. Haja cJavata. (From Hyrtl.) 



a.c, anterior cerebral artery; a.sp., spinalis artery; hs., basal artery; i.e., internal carotid 

 artery; m.c, median cerebral; ml., myelonal artery; p.c, posterior cerebral; tn., terminal 

 nerve. 



171a), which runs with the optic nerve to the eye. Each internal carotid then 

 turns upward, as in Heptanchus, and divides into the three cerebral arteries 

 which vary considerably in the different Elasmobranchs. 



The cerebral arteries, consisting of an anterior, a median, and a posterior 

 pair of arteries are, as we have seen in Heptanclms, derivatives of the right 

 and left internal carotids. The anterior cerebral arteries {a.c, fig. 172a) pass 

 forward ventrally around the lobi inferiores of the brain and over the optic 

 chiasma; in front of this, right and left arteries may be put into communica- 

 tion by a cross-trunk. From here forward great variation ensues. In some 

 types these arteries pass as single strands between the right and left divisions 

 of the telencephalon. In others they extend forward as numerous branches 

 {Heterodontus, rays). The median cerebrals (m.c.) extend under the telen- 

 cephalon and along the olfactory tracts as fairly simple vessels (Squalus 



