58 



with the left n. hypoglossus and was a branch of the a. carotis interna. 

 The a. basilaris arose from the carotid prior to its entrance into the 

 canalis caroticus of the skull . . . The a. vertebralis dextra was con- 

 tinued into the a. cerebelli inferior posterior. The a. vertebralis 

 sinistra, after its exit from the foramen transversarium of the first 

 vertebra is so narrow that it is not injected and becomes lost. The 

 a. basilaris after its emergence from the canalis hypoglossi, formed 

 a sharp flexure inwards, coursed in the middle of the clivus and was 

 thereafter perfectly normal. Batujeff thereafter gives the diameters 

 of the several vessels concerned in the abnormality with results gener- 

 ally resembling those of our own case and concludes by stating: "so- 

 viel ich mich überzeugen konnte, ist der von mir beschriebene Fall 

 der einzige in der Litteratur bekannte". 



These several cases just quoted form, it will be noticed, a pro- 

 gressive series. In Cavatorti's case the two aa. vertebrales remain 

 separate throughout. In Kendall's and Blackburn's examples there 

 has been partial union of the aa. vertebrales with subsequent separ- 

 ation; in our specimen there is total absence of union with one 

 vertebral replacing the a. basilaris, leading up to Batujeffs case of 

 total absence of union of the aa. vertebrales with an altogether ab- 

 normal origin of the a. basilaris. 



The development of the A. basilaris is but scantily dealt with in 

 the majority of the standard textbooks of Embryology; either no 

 mention is made whatsoever or the subject is summarily dismissed, 

 thus, the last edition of Quain's Elements of Anatomy (16), merely 

 states "the two vertebrals fuse below the hind-brain to form the basilar 

 artery. According to De Vriese, the two vessels become connected 

 by a network, and out of this a new mesial channel is developed". 



Hertwig's monumental "Handbuch der vergleichenden und experi- 

 mentellen Entwicklungslehre der Wirbeltiere" (17) describes a little 

 more fully the fate of the vessels in the several vertebrate groups but 

 for any real light on the morphology, phylogeny, and ontogeny of such 

 remarkable variations as this paper is concerned with it is necessary 

 to turn to the hterature of the subject. 



The two writers to whom we are most indebted for our knowledge 

 of the morphology, development, and phylogeny of the blood vessels 

 at the base of the brain are Beddard and Bertha De Vriese. 



Beddard (18, 19, 20) in a series of articles published in 1904/05 

 has made a very comprehensive study of the blood vessels at the 

 base of the brain in the lower mammals, sauropsida and aves. In 

 the sauropsida paper (18) Beddard points out that it is plainly 



