76 WwW. N. F. WOODLAND 
artery, when observed in longitudinal (Text-figs. 2, A, C, 
and 8, P) and serial transverse sections, is seen to possess in 
its course a number of regions in which its walls are very thick 
and its lumen therefore small. These thick-walled small- 
lumened regions constitute sphincters for the closure 
of the artery lumen, and each one of these sphincters 
is found to be situated immediately anterior to an autotomy 
plane (and behind the haemal arch of each vertebra) in the 
region of autotomy, and there is also one in front of the first 
autotomy plane (behind the last haemal arch of the unseg- 
mented base of the tail), as might be anticipated. When 
autotomy occurs at any segment it is the closure of the sphincter 
on the caudal artery immediately in front of this segment that 
prevents haemorrhage. As far as I am aware, this is the only 
instance yet described of a sphincter muscle bemg developed 
on a blood-vessel. (g) The caudal vein does not possess 
sphincters and this is not surprising, since the flow of blood is 
towards the body and therefore away from the portion of tail 
which is cast off. Nevertheless, to prevent undue loss of 
blood when autotomy occurs, the vein becomes constricted im 
the region of each plane of cleavage and dilates at each in- 
between region (Text-fig. 2, A, C), i.e. in the region of each 
haemal arch, and when the tail is shed the open lumen apparently 
becomes temporarily plugged up by blood-clotting. (h) Con- 
cerning the spinal cord there is nothing worth remarking, 
save perhaps that it contains as usual Reissner’s fibre (I have 
also observed this in the tail of Pygopus which is autotomous). 
It maintains an approximately uniform diameter throughout 
its course. On the ventral side of the spmal cord and in 
contact with its substance is a subneural vessel; also 
contained in the neural arch but lying external and ventral to 
the spinal cord are usually to be seen two lateral neural 
vessels, which in reality are part of a plexus of blood- 
vessels. 
The above-named structures are to be found in the segmented 
portion of the original tail of the Gecko. There remains for 
description the unsegmented base (Text-fig. 2, A) of the tail. The 
