40 



Messrs. Dreyer, Ray, and Walker. [Sept. 30, 



in connection with other work undertaken in the department in collaboration 

 with one of us (G. D.), but not yet published. 



All the animals used were strong and healthy and in good condition. 

 They had been kept in the laboratory under as equable conditions as 

 possible as regards food, etc., and, of course, had not been employed in any 

 previous experiment. Pregnant animals were naturally excluded. 



Methods. — The animals were killed, the aorta divided just above the 

 semilunar valves (care being taken that the section should be accurately 

 transverse), and a portion removed from the body by cutting through the 

 aortic arch after careful dissection. The proximal end of this portion of 

 the aorta was then measured in the following manner : — The severed artery 

 is placed in a special holder, closing with parallel limbs by means of which 

 a uniform and gentle pressure is applied to the vessel until its lumen is 

 just obliterated, and the walls meet in a straight line along the middle of the 

 closed vessel. This straight line is then measured with fine-pointed compasses 

 and the measurement pricked off on a piece of hard glazed drawing paper. 

 This length represents half the internal circumference (ttt) of the vessel. 

 The external measurement is recorded in a similar manner. The vessel is 

 then released completely and the process repeated again and again until from 

 four to eight independent measurements (both internal and external) have 

 been recorded. Great care is taken to avoid squeezing the vessel unduly. 

 At the same time sufficient pressure must be applied to ensure that the walls 

 lie quite flat against each other. The lengths thus pricked off upon the 

 paper form a permanent record, and are subsequently measured by means of 

 special callipers, fitted with a vernier scale which allows readings to be made 

 to 1/20 mm. From the observed irr the radius and the sectional area of the 

 vessel are determined. The internal measurements are those made use of in 

 the present communication, which deals only with the capacity of the vessel. 



It must be observed that while the measurements which we have made 

 give a reliable value for the (internal) circumference of the aorta within 

 each species, they do not necessarily afford an absolute measure of the 

 size of the vessel in these animals at any given phase of the circulation 

 during life. 



The method here made use of was selected because it proved to yield the 

 best results in our hands, particularly in the case of small animals. But it 

 is open to considerable experimental error. This source of fallacy, however, 

 is greatly reduced by multiplying the number of individual observations of 

 each artery. It also diminishes greatly with increasing experience. 



Own Observations. — In all the tables the body weight recorded is the 

 natural weight (in grammes) of the animal immediately before it was killed, 



