402 COMPARATIVE ZOOLOGY OF CHORDATES 



(adrenalin) has been synthetically prepared, but in spite of 

 this fact, little is known of the functions of the gland, except 

 that it produces effects similar to those due to stimulation through 

 the sympathetic autonomic nervous system. 



The Thymus. — The thymus first appears in the fish as a 

 series of paired upgrowths from the roof of the gill-slits. In 

 the Selachians it is more or less segmental in its arrangement, 

 but in higher forms the correspondence is lost, and the number 

 of slits which contribute to it is reduced. It controls the 

 formation of the shell, shell-membranes, and albumen in 

 birds' eggs. 



The Parathyroid. — The name parathyroid is given to 

 bodies which are usually situated close to or even in the thyroid, 

 but which differ from the latter in their structure and method of 

 development. They arise from the ventral regions of the 3rd 

 and 4th visceral pouches in the Tetrapods, and are apparently 

 absent in the fish. 



The Pineal. — The pineal eye has already been described in 

 connexion with the sense organs. In the higher vertebrates 

 this structure degenerates and is transformed into a gland. 



The Pancreas. — In addition to its function of producing 

 enzymes for the purpose of digesting the food in the intestine, 

 whither the enzymes are conducted by the pancreatic duct, 

 the pancreas also functions as an organ of internal secretion. 

 The tissue responsible for producing this internal secretion 

 is that known as the islets of Langerhans, and its production 

 is called insulin . The function of insulin is to store up glycogen 

 in the liver, in which respect it is antagonised by the adrenalin. 

 Diabetes is the result of faulty or non-functioning of the islets 

 of Langerhans. In some Teleost fish, the endocrine islet- 

 tissue may form little masses separate and apart from the 

 ordinary pancreatic tissue, which secretes the digestive pan- 

 creatic juice. 



The " Puberty " Gland. — The reproductive glands, ovary 

 and testis, in the birds and mammals produce internal secretions 

 which are concerned with the development and maintenance 

 of the characters which distinguish one sex from the other. 

 Since these secretions are essential for the proper sexual 



