289 



should establish the fact that the external carotid is primarily a 

 branch of the efferent hyoidean artery, or of the hyoidean aortic arch, 

 whichever it may be, and not of the dorsal aorta, it is evident, as 

 has been suggested at different times by myself (1908, p. 107) and 

 others, that the external carotid must either be developed from a 

 dorsal muscle branch of the hyoidean artery, or from a commissure 

 that primarily connected the hyoidean and some one or more pre- 

 hyoidean arterial vessels. Its origin from the lateral dorsal aorta, as 

 found in the adults of most fishes, would then be due to a simple 

 shifting of its point of origin upward on the hyoidean artery and 

 then forward or backward, as the case may be, along the dorsal aorta. 

 The internal carotid, or anterior prolongation of the lateral 

 dorsal aorta, can now be considered. This artery in Polyodon, after 

 its separation from the external carotid, runs forward, lying at first 

 immediately ventral to the outer edge of a short process of the 

 parasphenoid that projects laterally from the body of that bone 

 along the base of its ascending process. This short process gives 

 insertion to a strong ligament that embraces the ventral edge of 

 the anterior portion of the pseudobranch, and from there runs back- 

 ward external to the spiracular canal and has its insertion on the 

 ventro-anterior edge of the hyomandibular. The protractor hyomandi- 

 bularis muscle lies immediately anterior and parallel to this ligament, 

 and a small posterior head of the muscle has its insertion on the side 

 wall of the skull immediately anterior to the ascending process of 

 the parasphenoid, the surface of insertion lying ventral to a large but 

 shallow groove that marks the course of the jugular vein and external 

 carotid artery as those two vessels run forward after issuing from the 

 anterior opening of the facialis canal. The internal carotid, running 

 forward from its point of origin, lies ventral to the ligament and 

 muscle just above described, and in this part of its course is closely 

 accompanied by the efferent pseudobranchial artery and by a pharyngeal 

 branch of the nervus glossopharyngeus. The internal carotid here 

 becomes surrounded by a dense fibrous tissue which is attached to 

 the latero-ventral angle of the side wall of the skull, the tissue forming 

 a tube which encloses the artery and separates it from the accom- 

 panying nerve and efferent pseudobranchial artery. Anterior to 

 the surface of insertion of the posterior head of the protractor 

 hyomandibularis, this connective tissue tube becomes continuous with 

 the edges of a groove on the ventral surface of the lateral etlge of 

 the base of the chondrocranium, the chondrocranium here projecting 

 laterally beyond the lateral edge of the parasphenoid, as shown in 

 Bridge's figures 2 and 4. The efferent pseudobranchial artery here 



Anat. Anz. Aufsätze. XXXIX. 19 



