426 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



The latter divides and sends a branch backwards and another for- 

 wards along the outer dorsal surface of the mandible. 



The A. carotis interna (PI. VIII. Fig. 1, c. in.) arises from the 

 anterior surface of the first branchial vein close to the carotis externa, 

 and passes forwards along the side of the skull. A short distance from 

 its origin it thickens into a gland-like structure (p s) nearly 8 mm. long 

 and 3 mm. wide in the middle and tapering towards both ends. This 

 organ is exposed from the roof of the mouth by dissecting away the 

 adductor arcus palatini from its attachment to the side of the skull. 

 From this surface (ventral) the channel of the artery is distinctly 

 seen passing directly through it from end to end Transverse sec- 

 tions show that the wall of the vessel is thickly perforated throughout 

 the length of the organ by small openings of vessel-like passages 

 (Fig. 2, b), which are quickly lost in the fine interspaces of the con- 

 nective tissue of which the thickening is formed. Scattered through 

 it are seen the small arteries by which the blood is again collected 

 from the interspaces (Fig. 3, a). An examination of the position and 

 relations of this structure leaves no doubt but that it is the remains 

 of the pseudobranchia which has become reduced to a mere rete 

 mirabile. It is worthy of note in connection with its reduced state 

 in Amiurus, that Owen mentions Silurus as one of those fish in 

 which it is entirely absent. That it is the pseudobranchia is shown 

 bv the fact that the arteria ophthalmica magna (Fig. 1 a. o. m,) arises 

 from its anterior dorsal surface which is in contact with the optic 

 nerve, in company with which the artery passes to the eye. 



Three small arteries arise from the same surface, posterior and 

 medial to the former, and immediately enter the braincase. These 

 are the encephalic arteries, and their origin from the pseudobranchia 

 is unknown among other Teleostei. 1 In this point, however, as also 

 in the structure of the organ Amiurus shows a singular agreement 

 with Acipenser. 2 



As far as I am aware the pseudobranchia has not the peculiar 

 direct relation to the carotid, described above, in any other fish, but 

 is situated upon a branch of that vessel even in the sturgeon. 



The internal carotid supplies the adductor arcus palatini, a branch 

 to the posterior part arising behind, and those to anterior part after 



1 Dr. F. Maurer — Eia Beitrag z. Kennt. d. Pseudobranchien d. Knochenflsche. Morph.. 

 ahrb. Bd. IX. Taf. XI. 



Owen— I. e. Vol. I. p. 489. 



