530 A MANUAL OF PHYSIOLOGY 



volume of the urine passed in each interval of twenty four hours 

 being known, the total excretion of urea for the twenty-four hours 

 can be calculated, and a curve plotted to show how it varies during 

 the period of experiment.* 



5. Thyroidectomy. Study the anatomy of the neck and the rela- 

 tions and blood-supply of the thyroid glands in a dog used for some 

 previous experiment. 



(1) Then select a half-grown dog, weigh it, inject morphia subcu- 

 taneously (p. 1 76), and fasten on the holder back down. Clip the hair 

 from the neck, and shave a wide space on each side of the middle 

 line. Scrub with soap and water, then with corrosive sublimate solu- 

 tion (o-i per cent.). Sponges, instruments, ligatures, etc., must have 

 been boiled in water; the instruments are immersed in 5 per cent, 

 carbolic acid solution, everything else in the corrosive solution. The 

 hands and nails must be carefully cleansed and washed with the cor- 

 rosive sublimate. A longitudinal incision is made through the skin and 

 subcutaneous tissue in the middle line of the neck, beginning a little 

 below the projecting thyroid cartilage. By separating the longitudinal 

 muscles just external to the trachea on one side, the corresponding 

 thyroid lobe will be seen as an oval red body. It is now to be care- 

 fully freed from its attachments ; all vessels connected with it are to 

 be tied with double ligatures, and divided between the ligatures. In 

 tying the superior thyroid artery (a short large vessel coming off from 

 the carotid), care must be taken not to put the ligatures too near its 

 origin, as the rapid current in the carotid may prevent closure of the 

 vessel by clot, and secondary haemorrhage may occur some days after 

 the operation. The thyroid lobe is thus shelled out of the tissues in 

 which it lies. If, as rarely happens, an isthmus is present (connecting 

 the two lobes across the front of the trachea), it must also be removed. 

 All bleeding having been stopped, the wound is washed out with 

 corrosive solution, and the muscles brought together over the trachea 

 by a row of interrupted sutures, which should not be drawn too 

 tight. The wound in the skin is then closed by a similar row, 

 preferably of subcutaneous sutures (see p. 190). Collodion is 

 painted over the wound, and the animal is returned to its cage. It 

 should be kept for a week, or, better, a fortnight, and examined care- 

 fully during that time. Probably, unless the wound has become 

 infected, its behaviour will be perfectly normal. 



(2) The second part of the experiment, which consists in removing 

 the remaining thyroid lobe, is now to be performed just as in (i). The 

 animal must be examined next morning, and then twice a day for the 

 following week, as the symptoms of cachexia strumipriva generally 

 come on very rapidly in young dogs, and death may even ensue 

 within two days. Trembling of the limbs, associated with instability 

 of movement, spasms resembling those of tetany, sometimes passing 

 into general epileptiform convulsions, and progressive emaciation, 



* In 17 healthy students the average amount of urea excreted in twenty- 

 four hours on the ordinary diet was 29*51 grm. (minimum 19*35 & rm< > 

 maximum 46*007 grm.) ; on a diet poor in proteid, average 2075 grm. 

 (minimum 9*517 grm., maximum 32-857 grm.) ; on a diet rich in proteid, 

 average 38*83 grm. (minimum 23*265 grm., maximum 67-82 grm.). 



