386 USES OF THE LIVER DUCTLESS GLANDS 



Vessels and Nerves. The bloodvessels of the thyroid gland are 

 very abundant, this organ being supplied by the superior and inferior 

 thyroid arteries with sometimes a branch from the innominata. The 

 arteries break up into a close capillary plexus, surrounding the vesicles 

 with a rich network, but never penetrating their interior. The veins 

 are large, and like the hepatic veins, they are so closely adherent to the 

 surrounding tissue that they do not collapse when cut across. The 

 veins emerging from the gland form a plexus over its surface and the 

 surface of the trachea, and they then go to form the superior, middle 

 and inferior thyroid veins. The nerves are derived from the pneumo- 

 gastrics and from the cervical sympathetic ganglia. The lymphatics 

 are abundant but difficult to inject. The exact distribution of the nerves 

 and the origin of the lymphatics are not well understood. 



Nearly all that is known in regard to the chemical constitution of 

 the thyroid may be embodied in the statement that it contains leucin, 

 xanthin, lactic acid, succinic acid and some volatile fatty acids. Recently, 

 however, a substance called iodothyrin has been extracted, which contains 

 nearly ten per cent of iodin in combination with proteids. It is prob- 

 able that this is the active principle of thyroid extract, now often used 

 in therapeutics. The blood of the thyroid veins has been analyzed, but 

 the changes in its composition in passing through the gland are slight 

 and indefinite. It has been said that one of the uses of the thyroid is 

 to regulate the blood-circulation in the brain, but the observations in 

 support of this view are not satisfactory. 



Myxcedema. Important clinical facts have been developed showing 

 a connection between the thyroid gland and a disease characterized by 

 infiltration of the connective tissues with a gelatinous substance con- 

 taining mucin. This disease has been described by Ord under the 

 name of myxoedema. It is attended with marked impairment of the 

 mental faculties and a condition like cretinism. This usually is asso- 

 ciated with disease of the thyroid gland. 



Complete excision of the thyroid gland in the human subject has 

 been followed by the peculiar mental condition characteristic of cretin- 

 ism. In the lower animals the operation of complete extirpation is fatal. 

 The experiments of Horsley on dogs and monkeys show great differences 

 in the results, depending on age. In young animals death usually occurs 

 in a few days, while old animals survive the operation four, five or six 

 months. So far as could be ascertained from these experiments on the 

 lower animals dogs and monkeys the conditions, including the 

 mental phenomena, resembled those observed in cases of myxoedema 

 in the human subject. The animals operated on were found to be 

 exceedingly sensitive to cold ; but when put in a hot-air bath at a tern- 



