372 THE AMERICAN NATURALIST. > (VoL. XXXIX. 
The precardinal veins, however, are rarely injected by this or by 
any other method, except that of direct injection of each pre- 
cardinal from its anterior end, —an operation which is not 
practicable for anyone who is not familiar with the position of 
the vein. 
The filling of the cavities of the heart may be prevented or 
controlled by means of ligatures tied around the heart in the 
regions of the sinu-auricular and auriculo-ventricular apertures. 
The injection into the posterior division of the lateral vein fills 
the posterior connections of that vein, including the veins from 
the pelvic fin (see Rand and: Ulrich, :05). 
In brief, the advantages of injection va the lateral vein are as 
follows: (1) the lateral vein is a sufficiently large vessel and 
most easy of access; (2) its preparation for injection is a simple 
operation, requiring no nice dissection nor other delicate tech- 
nique ; (3) it is a relatively narrow and strong-walled vein, not 
easily broken by the.point of a cannula, and capable of with- 
standing the initial pressure of the injection ; (4) it conveys the 
mass first to the precaval sinus, whence the chief vessels may 
ll ; (5) the weak-walled cardinal sinus is remote from the point 
of injection; (6) maximum filing of the systemic veins is 
secured by a single operation. 
An injection of the renal portal system is secured easily by 
cutting across the tail at least four or five centimeters back of 
the cloacal aperture and injecting forward into the caudal vein. 
A taper-pointed glass cannula may be inserted carefully into the 
cut end of the vessel and pushed in until the end of the vein is 
tightly closed. During the progress of the injection the can- 
nula must be held firmly in place.. When the cannula is with- 
drawn, the escape of the fluid may be. poene by jamming a 
bit of cotton into the end of the vein. 
The hepatic portal system is best injected through the mesen- 
teric vein. To secure a full injection upon the intestine the vein 
must be injected backward as well as forward. Parker ('95, 
P- 48) injects the duodenal vein, thus getting a complete injec- 
tion at one operation. This is, indeed, an advantage. But, in 
small skates, the greater size of the mesenteric vein, and the 
fact that it is so much more accessible than the duodenal vein, 
