154 PRINCIPLES OF ANIMAL BIOLOGY 



retained in a dilatation of the lumen of the gland until the secretion is 

 required. When the gland is stimulated in the appropriate manner the 

 secretion is poured out as a result of contraction of the muscles of the 

 gland. In many other glands the secretions are retained within the cells 

 and are not discharged until the gland is stimulated. Stimuli which 

 rouse glands to activity are of various sorts, the chief of them being ner- 

 vous and chemical (by means of hormones). The action of a hormone 

 has been cited in the account of the stimulation of the pancreas by secretin. 

 This substance is produced from a material in the cells of the duodenum 

 by the action of the acid chyme. The secretin is absorbed by the blood 

 and is carried to the pancreas where it stimulates the cells to discharge 

 their secretions. Control by the nervous system is discussed below. ^ 



Glands of Internal Secretion. — Glands which have no ducts and whose 

 secretions must diffuse (or are emptied) into the lymph and blood are 

 called glands of internal secretion. In this category may be included the 

 pituitary, thymus, thyroid, and adrenal glands. Some of the glands 

 provided with ducts also produce one or more secretions that are ab- 

 sorbed by the blood and so these glands must be regarded as glands of 

 internal secretion. These are the pancreas, liver, and sexual glands. 

 The last mentioned glands maintain the germ cells, but thej^ also produce 

 some secretions which are absorbed by the blood and have a most pro- 

 found influence upon physical and mental processes. The functions of 

 some of these glands of internal secretion are coming to be known through 

 experiments with removal of the glands and with feeding the extracts 

 of glands. Some of their secretions influence blood pressure; others affect 

 muscular contraction and muscular tonus; others control the growth of 

 skeletal parts and of tissues, the growth of primary and accessory sexual 

 organs and development of secondary sexual characters; and some 

 influence nutrition and other metabolic activities. A study of internal 

 secretions figures largely in the training of the medical investigator, 

 but a further discussion of them would be too involved for this book. 



Diversity of Glandular Secretions. — A considerable number of secre- 

 tions have already been named and something of their functions pointed 

 out, but they form only a beginning of the list that might be given. 

 Among the secretions that may not immediately occur to the student as 

 belonging to this class of substances are the sebaceous materials which 

 make the hair and skin oily, milk for nourishment of young , wax such as 

 beeswax, poisons like those of snakes and certain insects, acids such as 

 formic, sulphuric, and hydrochloric, gas as in the swim bladder of fishes 

 and the float of the Portuguese man-of-war, the ink-like sejMa which is 

 thrown out to cover up the escape of the cuttlefish, silk of silkworms and 

 other insects and of spiders, the odors of insects by which they attract 



' Reference should here be made to the description of the morpliology of glands in 

 the preceding chapter, where many facts regarding function are also given. 



