1905.] ARTERIAL SYSTEM IX SAUEOPSIDA. 63 



basilar ai-tery are tlie posteiioi- cerebellar, and these arise a little 

 behind the middle of the medulla. The two arteries are perfectly 

 symmetrical with each other as to their point of origin. They 

 are, however, different in their branching. The light artery 

 gives off, shortly after its oi-igin from the basilar, a strong artery 

 running backwards along the side of the spinal cord. This 

 branch exists and pursues the same course on the left side ; but 

 on that side of the brain it arises separately from the basilar 

 artery. Between the origin of the posterior cerebellar arteries and 

 the bifurcation of the basilar anteriorly are three pairs of small 

 arteiies supplying adjacent regions of the medidla. A slightly 

 larger artery, which is the anterior cerebellar, arises from the 

 fork of the basilar. This fork is U-shaped in the Teguexin (text- 

 fig. 18, p. 65), and not V-shaped as in the other Lizards desciibed 

 here. The U -shape is due to the fact that the two carotids run 

 parallel to and almost in contact with each othei- for some distance 

 before they join the circle of Willis. The carotids, moreover, lie 

 within the area bounded by the third nerves very close to and about 

 on a level with those nerves. The artery formed by the junction of 

 the basilar and cai'otid on each side, often spoken of merely as the 

 carotid, passes outwards and slightly backwards at first, when it is 

 practically at right angles with the basilar. In this I'egion the 

 artery shows diflerent relations on the tAvo sides of the body. On 

 the left side it runs in front of the third nerve ; on the right side 

 it lies behind that nerve. The first branch arising after the 

 carotid is at the bend of the artery, where it turns forward ; this 

 very stout arteiy suj)plies the cerebellum and optic lobe ; im- 

 mediately in fi'ont of this is the artery of the optic lobe. This 

 state of aflairs occurred on the left side of the body ; on the right side 

 the two arteries arose by a common trunk. On both sides the 

 artery of the corpus bigeminum gives off an artery to the cerebral 

 hemisphere which buries itself in the furrow between the hemi- 

 sphere and the optic lobe. From the inner side of the circle of 

 Willis, just opposite to the bigeminal artery on the left side and to 

 the conjoined arteries just mentioned on the right side, arises an 

 artery which runs to the optic chiasma. This artery is precisely 

 like that of other Lacertilia. The next artery to be given ofi" is 

 the posterior cerebral, Avhich plunges at once into the furrow lying 

 between the optic lobe and the cerebral hemisphere. The middle 

 cerebral artery, which is the largest of the cerebral arteries, runs 

 in the usual way along the Sylvian depression, and just in front 

 of the point of origin of this the circle of Willis practically ends 

 in the strong ophthalmic arteries which follow the optic nerves. 

 There are therefore no difterences of importance betw^een the 

 ai-terial system of the bi-ain of Tujnnamhis and of the other 

 genera of Lizards reported iij^on in the present communication. 



§ Cerehral Arteries m the Lacertilia. 

 We may deduce from the facts just described the chief 



