362 MISS JOAN B. PROCTER ON THE NUMBER OP 



readied this point, it takes place from the neck hachmrds until 

 complete. 



In O. austriaca the IVth, which is in process of reduction, 

 halts at the same point, somewhere on the third fourth of the 

 body, and when further reduction is required the whole series 

 commence to break up by intermittent fusion with its neighbour. 



It is thus obvious that the portion of the trunk-proper where 

 the skin receives the most strain is that anterior to this h.alting- 

 point, the greatest strain of all being on the middle third. The 

 reason for this is the presence of the stomach, a fact which I have 

 verified by the dissection of 15 spirit-specimens*, confirmed by 

 the dissection and special study of three fresh specimens, taking- 

 particular notice of the relative positions of the pyloric sphincter, 

 and the point where the final loss occurs in the involved scale- 

 rows. In every case the halting-point is on a level with the 

 pyloric sphincter, or some Avay posterior to it ; in no case does it 

 occur anterior to the pylorus. To test this I took a small viper 

 enormously distended with food (spirit specimen) and stuck a pin 

 into it where it first regained its normal girth. On examination 

 this pin pi'oved to have been lodged witliin a venti'al of the point 

 where the IVth series fused with the Ilird, on the left side ; 

 dissection proved this point to coincide also with the pyloric 

 sphincter ; the stomach-proper in this case was distended by a 

 lai'ge mouse. Most cases are of coui'se less exact, as there is much 

 individual variation, the given points usually varying a little on 

 the two sides of the body. 



If one considers a snake's manner of feeding, and the length of 

 time that the stomach remains distended, it is not surprising 

 that the reduction of scale-rows should be held up where the 

 strain on the skin is so great, or that new rows should commence 

 by out-cropping at this region. 



Dr. J. 0. Thomson, in his paper on ophidian anatomy f, has 

 studied this question of the variation in the number of scale- 

 rows, together with the external landmarks of the principal 

 organs, the relative positions being given in term of ventrals. 

 Of ThamnopMs ordinokles he says: — "The relation existing be- 

 tween the position of the viscera and the added and suppressed 

 scale-rows has been studied in fifty specimens of this species." 

 " 19-21-19-17, that is 19 rows forward, increasing to 21 at about 

 the level of the heart and continuing to the end of the liver, 

 then decreasing to 19, and further on to 17." He does not, 

 however, mention the stomach and its physiological bearing on 

 the question, nor does he record its position in the tables of 

 " The external landmarks of the principal viscera." 



The high number of rows behind the head can also be accounted 

 for, this area of skin being subjected to the greatest strain of all, 

 by the enormous distension of the jaws in the struggle to swallow 



* 4: T. natrix, 5 C austriaca, and 6 V. berns. 



t " Further Contributions on the Anatomy of the Ophidia." Proc. Zoal. Snc, 1914, 



