60-i VETERINARY PHYSIOLOGY 



thyreoid. Supplementary parathyreoids are frequent, and in 

 some animals, e.g. cats, they are embedded in the thymus. It 

 is therefore impossible to be sure that, after removing the usual 

 four parathyreoid bodies, a considerable amount of parathyreoid 

 tissue is not left. This explains the negative results got by 

 some experimenters. 



3. Structure. — Each consists of columns of cells (a) the chief 

 cells, large and not staining readily, (6) oxyphil cells, smaller 

 than the last, and with granules staining with eosin. Masses 

 of colloid material may occur, giving the structure somewhat 

 the appearance of the thyreoid gland (fig. 229). 



4. Physiology. — (1) Removal. — Since 1882 it has been known 

 that, after removal of the thyreoid gland for goitre in the human 

 subject, a peculiar condition of spasm of the muscles, and even 

 of convulsions leading to death, may occur. This was first 

 ascribed to removal of the thyreoid, but in 1896 Vassale and 

 Generali definitely showed by experiments upon animals that it 

 is due to removal of the parathyreoids, and that, if a sufficient 

 amount of their tissue is left, the symptoms do not develop. 



The symptoms are (i) depression and emaciation ; (ii) tonic 

 contraction of various muscles, chiefly the extensors ; (iii) 

 tremors and jerkings of the muscles, which may go on to 

 a general convulsion. Sometimes these disturbances of the 

 neuro-muscular system are accompanied by disturbances of 

 balancing. (iv) A peculiar increase in the excitability 

 of the peripheral motor neurons, so that if a motor 

 nerve is compressed or tapped a violent convulsive movement 

 of the muscles supplied by it may be produced, while the 

 response to galvanic stinmlation is enormously heightened, 

 (v) In the dog increased rate of the heart and of the respirations. 

 The symptoms vary greatly from time to time, and may remain 

 latent except for the increased excitability of the nerves which 

 persists. 



The spasticity and tremors are due to the implication of the 

 motor neurons of the spinal cord and are arrested by cutting 

 the nerve to the muscles. The increase in the response to 

 the stimulation of peripheral nerves is due to an increased 

 excitability of the nerve endings in the muscles. These two 

 conditions are not necessarily proportional to one another. 



All the symptoms are due to a poison developed in the 



