490 



branchial aiterv, forms the " Eamus anastomoticus s. communicans 

 zur Carotis interna" of Hyrtl's (1872) descriptions of selachians. 

 Parker, in Mustelus antarcticus, describes a so-called ophthalmic 

 artery that has its origin from his anterior carotid and goes to the 

 eye-ball, but he neither describes nor figures an artery arising from 

 the anterior cerebralis (internal carotid) artery and also going to the 

 eye-ball; that is, he describes the arteria ophthalmica magna of my 

 nomenclature but not the optic artery. Hyrtl, in Mustelus plebejus, 

 describes both these arteries, the optic artery being called by him the 

 arteria ophthalmica and said to go to the eye-ball, while the ophthal- 

 mica magna is considered as a branch of the internal carotid that is sent 

 to the orbit, and not to the eye-ball, and there anastomoses with 

 his so-called Ramus anastomoticus. In Scyllium canicula Hyrtl 

 gives the former one of these two arteries but not the latter, while in 

 Acanthias vulgaris he gives the latter artery and not the former. 

 These several descriptions should evidently be controlled. 



Both, Parker and Hyrtl, in the selachians described by them, 

 show the internal carotid arteries of opposite sides crossing in the 

 median line, in the pituitary region, and there each going definitely' 

 to the opposite side of the head. Parker does not mention an anas- 

 tomosis of the arteries at the point of crossing, but Hyrtl describes 

 it. In Heptanchus the internal carotids meet and fuse in the median 

 line, in this same region, but there is no indication of a crossing from 

 one side to the other; and in Chlamydoselachus the arteries certainly 

 do not here cross, for they remain at a certain distance one from the 

 other and are connected by transverse commissure. In Chlamydo- 

 selachus, however, each internal carotid here forms a short, close loop, 

 by turning mesio-dorso-posteriorly and then latero-anteriorly upon 

 itself, and it is the summit of this loop, which is directed mesially, 

 that is connected with its fellow of the opposite side by the commissure 

 above referred to. This loop is apparently simply the result of blood 

 pressure in an elastic vessel that turns abruptly dorsally and is firmly 

 held at the point of turning, and if the summits of the loops of oppo- 

 site sides were to approach and fuse it might give the appearance 

 of a crossing of the arteries. Wishing to see if this might account 

 for the conditions described by Parker and Hyrtl, I have examined 

 two sharks' heads that I had in my laboratory; one a head of Galeus 

 of unknown species and the other unidentified and not classifiable. 

 In Galeus the internal carotids enter the base of the skull by separate 



