GENETIC TYPE AND THE ENDOCRINES 401 



THE RELATIVE SIZES OF THYROID GLANDS IN CONTRASTED 

 TYPES OF PURE DOG BREEDS 



There are usually two distinctly separate thyroid lobes in 

 the dog. These are situated under cover of the ventral neck 

 muscles, bilaterally on the right and left sides of the trachea 

 a short distance caudal to the larynx. In some cases the two 

 lobes are connected by an isthmus of thyroid tissue passing 

 ventrally across the trachea. The isthmus varies in extent 

 from a narrow, almost transparent strand of tissue to a 

 strong wide band of thyroid material resembling to a mild 

 degree the isthmus of the human thyroid. In our experience, 

 the isthmus is found more often in the flat muzzled bulldog- 

 like breeds than in the long jawed types. Several English 

 bulldogs have shown the wide, strongly developed thyroid 

 isthmus, and the flat faced Pekingese and its hybrids fre- 

 quently possess a thin isthmus. However, this is not a con- 

 sistent finding, and the bulldog may lack an isthmus entirely, 

 while a long muzzled breed, such as the German shepherd, 

 may possess one, though such cases are rare. 



Also occurring in the dog is an additional mass of thyroid 

 tissue in a more posterior position. Kampmeier ('37) has 

 recently recorded the development of thyroid material in 

 the region of the aortic and pulmonary arches in several 

 mammals. Nonidez (unpublished results) has found thyroid 

 tissue w r ithin the pericardium of a few of the puppies used 

 in these experiments. And James, working at our kennels, 

 has observed that after complete removal of both thyroid 

 bodies from the neck region of the dog, the usual subsequent 

 physiologic changes failed to develop in certain individuals, 

 while others showed the expected ill effects. The animals 

 showing no symptoms of thyroid deficiency after what was 

 considered complete thyroidectomy were found to contain 

 thyroid tissue in the mediastinum. 



The volume of thyroid tissue from a large number of pure 

 bred dogs and their hybrids was measured in both the fresh 

 state and after fixation and, for comparative purposes, the 



