Glands 113 



the ductless glands. 



Internal secretion is the production of hormones; substances which 

 certain cells discharge into the blood, and which are carried by the blood 

 stream to other cells, upon which these substances exercise a specific action. 

 A certain hormone (secretin) we have seen is produced by epithelial cells 

 in the duodenum; others are probably produced by cells in the epithelium 

 elsewhere, and possibly by muscle cells. 21 Certain glands, as for example, 

 the pancreas, produce both an external secretion and an internal secretion : 

 the hormone produced by the pancreas has a definite effect on nutritive 

 processes throughout the body. 



There is one class of glands conventionally designated as ductless 

 glands, which produce internal secretions only. The principal members 

 of this class are : the two adrenal glands (suprarenal capsules or adrenal 

 bodies) lying near the upper end of the kidneys; the thyroid (or thy- 

 reoid) gland (or body) which partly surrounds the upper part of the 

 trachea (windpipe) and the pharynx [Fig. 81] ; the parathyroid (or 

 parathyreoid) glands of which there are usually two pair lying near the 

 thyroid; the pituitary body (hypophysis)- 2 lying in front of the brain 

 stem [Figs. 66 and 67] ; the pineal gland (or body) just above the cor- 

 pora quadrigemina ; the carotid body, at the bifurcation of the carotid 

 artery; the coccygeal body, in front of the tip of the coccyx (the terminal 

 vertebrae of the spinal column) [Figs. 29, 38] ; and the thymus [Fig. 

 81] in the lower part of the neck and upper part of the thorax. 23 



The thyroid, the parathyroid, and in part the pituitary body have 

 alveolar structure: i. e., they contain lumina or follicles whose walls are 

 composed of secreting cells ; but these follicles have no ducts. The other 

 ductless glands are rather of the solid type: the cells being bunched in 

 masses or columns between which the blood vessels and lymphatic vessels 

 ramify. 



The hormones from certain glands enter the blood-stream directly. 

 From some glands they enter wholly (thyroid) or partly (pituitary) 

 through the lymph channels. 



21 According to Howell, the liver cells produce two internal secretions — glycogen 

 and urea. The former is conveyed to and consumed by muscle cells throughout the 

 body; the latter is excreted by the kidneys. These substances, according to Starling, 

 do not properly fall in the class of hormones; hormones are strictly substances which 

 have a stimulating or sensitizing effect, as the derivation of the term hormones indi- 

 cates. 



22 See footnote, Pg. 83. 



28 The ovaries, testicles, spleen, and the lymph-nodes are sometimes classed as duct- 

 less glands. It is probable that these bodies produce hormones, but if so they are 

 not their principal products. 



