46 BULLETIN OF THE — ’ 
and hence essentially unpaired, —a view which Hoffmann in Bronn’s 
Thierreich, Reptilia, unfortunately adopts. 
Van Bemmelen (388, pp. 114, 115) combats the view of Fritsch, and 
brings forward additional instances of the persistence of the right proxi- 
mal root of the a. subvertebralis in embryos. At the same time he 
points out that Rathke’s a, subvertebralis must be regarded as equiva- 
lent to the dorsal collecting trunks of the anterior branchial vessels, and 
therefore to the aa. carotides interne of Rathke’s general scheme. The 
vessels designated in my description above by the numeral 2 — called 
by Rathke a. collateralis colli — are, on this hypothesis, to be regarded 
as aa. carotides externe (ventral distributing trunks of anterior bran- 
chial vessels). 
Mackay (’89, pp. 126-136) more than any other has contributed by 
his embryological and comparative anatomical studies to an interpreta- 
tion of the carotids of the Crocodilia. The vessels a and 6 are, accord- 
ing to him, the parts of the ventral distributing trunks which lie between 
the fourth and third arches, and correspond to the common carotids of 
Rathke’s general scheme. The morphologically paired vessels (1) are the 
combined internal, or, better, “ dorsal” carotids. The part designated in 
Figure 1 by 1* is thus homologous with the third visceral (first branchial) 
vessel. The part designated by 1° + 1” has arisen by fusion of the dor- 
sal collecting trunks of the three anterior branchial vessels through a 
part of their extent. The vessels marked 2 and 2’ (Fig. 1) are external 
or ‘‘ ventral” carotids, — these vessels being represented in Birds also, 
where the so called common carotids are in reality “ dorsal” carotids, 
not equivalent to the common carotids of Lizards. Mackay’s results, 
which thus confirm and extend van Bemmelen’s, seem conclusive, not 
only because he has traced the development of the homologous vessels 
in Birds, but because he has found one instance — like that of Brandt 
(72, p. 307) long ago —in which the dorsal collecting trunk persists 
between the third and fourth arches. It is connected with the third 
arch near the proximal (posterior) end of the a. subvertebralis, and is 
thus far removed from Rathke’s so called ‘a. carotis interna” of the 
head region. The hypothesis that the a. collateralis colli of Crocodiles 
is homologous with the a. carotis externa of Lizards, receives additional 
support from the fact that, as in Birds, the a. subclavia arises near the 
point at which the a. collateralis colli is formed by the division of the 
true a. carotis communis (@ and 1). 
The conclusions of Mackay have been recently confirmed by the re- 
searches of Hochstetter (’90). 
