the Superior Vena Cava of the Cat 229 



Whilst investigating this point we began another series of 

 observations on the Superior Vena Cava itself, which will perhaps 

 be of more general interest than the point of anatomical detail 

 just described. 'This work is still proceeding, but the results already 

 obtained are perhaps of sufficient interest to justify a brief summary 

 of them. 



Of the large changes in the circulatory system by far the most 

 recent is that from the condition of two Superior Venae Cavae to 

 that of one Superior Vena Cava, and the latter condition has ap- 

 parently been evolved independently in more than one order of 

 the Mammaha. The nature of the variation in the single Vena Cava 

 would therefore appear to have considerable interest. Cases of 

 two Superior Venae Cavae have, we believe, been recorded for the 

 human subject, and at least one such case is recorded for the dog. 

 The Superior Vena Cava of the cat is not only large but long, being 

 normally, as we shall see in a minute, almost half as long as the 

 trachea. It is normally about four times as long as the Innominate 

 Veins, which are, therefore, very short vessels running almost at 

 right angles to it. We decided to measure the variation in the 

 length of the Vena Cava, taking as our standard the length, of the 

 trachea from the posterior edge of the cricoid cartilage to its 

 bifurcation into the two main bronchi. The trachea proved to bear 

 a fairly constant relation to the total length of the animal, exclu- 

 sive of the tail, but we checked our standard by making other 

 measurements. The length of the trachea varied from 21% of 

 the total length to 26-7%. To summarise the results, we found it 

 necessary to exclude the kittens from the scheme as they showed 

 in many respects very anomalous relations between the sizes of 

 their different organs. This left us with only twenty-one specimens, 

 all these being over 40 cms. in length. Taking the length of the 

 trachea as 100, the extreme limits of variation in the length of the 

 Vena Cava were 19 and 47-9. But the majority of the cats are by 

 no means massed about the mean between these extremes. On the 

 contrary, the great majority, 16 out of 21, are massed close to the 

 upper limit. These 16 varied from 38 to 47-9. This is obviously the 

 normal type and in it the Vena Cava is approximately four times 

 the mean length of the Innominates. And it may be said that in 

 this type the blood coming from the fore-limbs and head runs 

 into a single channel at the earliest possible moment. There was 

 one case in which the Vena Cava sank to just under 35 %, but 

 which, nevertheless, probably belongs, we think, to the normal 

 category; because this was the cat which had the longest trachea 

 (26-7 %), and if the Vena Cava be judged either by the total length 

 of the animal or by the Innominates, it comes up to the normal 

 length. Then there are three other cases which varied from 29 % 

 to 33 %. These are very possibly a different type, but the numbers 



