332 FISHES CHAP. 
branchial arteries of the two sides the dorsal aorta is formed. It 
may be pointed out that the anterior efferent vessel of each arch, 
which is usually larger than the posterior one, is to be regarded 
as the primary efferent artery of the corresponding holobranch, 
and as such is directly continuous with an epibranchial artery, 
the posterior efferent artery being a secondary vessel which opens 
not into the primary trunk of its own branchial arch, but into 
that of the succeeding arch.’ The principal arteries which 
supply the various parts of the head with blood are derived 
from the first efferent branchial vessel. From the ventral end 
of this artery a mandibular artery is given off, which subdivides 
into branches for the muscles of the lower jaw as well as into 
nutrient vessels for the hyoidean hemibranch. At about the middle 
of its length the same artery gives off an afferent pseudobranchial 
or hyoidean artery, to the spiracular or mandibular pseudobranch. 
From the latter organ the blood is collected by an anterior 
carotid artery which, after giving off an ophthalmic branch to 
the eye, perforates the orbital wall and enters the cranial cavity, 
where it is joined by an anastomotic trunk from the posterior 
carotid of the opposite side; finally, the anterior carotid divides 
into anterior and posterior cerebral arteries for the brain. The 
third and last of the cephalic arteries is the posterior carotid ; 
this artery arises from the dorsal extremity of the first efferent 
branchial vessel, and, on entering the orbit, gives off the anosto- 
motic trunk previously mentioned. The latter vessel enters the 
cranial cavity, and, after crossing its fellow, joins the anterior 
carotid of the opposite side, as described above. The main trunk 
is then continued forwards in the orbit, and its various branches 
eventually supply the eye-muscles, the mandibular adductor muscle, 
and some other parts of the head. 
It is worthy of note that the median dorsal aorta is prolonged 
forwards in front of the first pair of epibranchial arteries as a 
slender median vessel (a.d.a), which ultimately divides into two 
branches, each branch uniting with the posterior: carotid of its 
side. 
A remarkable system of arteries for the supply of nutrient 
blood to the gills and heart has its origin in the following 
' Chlamydoselachus is more primitive in this respect, and has but a single 
efferent vessel for the two hemibranchs of each arch, which corresponds with the’ 
more anterior of the two in Mustelus (Ayers, op. cit.). 
