102 ; ; A INTRODUCTION TO ZOOLOGY 



away by ducts, these structures do not possess a free surface nor a 

 duct to convey off the secretion. They are so obviously glandular 

 in structure that their activity has been generally assumed, and as 

 they are plentifully supplied with blood-vessels, it was also assumed 

 that their secretion must be passed directly into the blood stream, and 

 in consequence it is often termed an Internal Secretion. Although 

 they have long been known, it is only comparatively recently that 

 the great importance of some of these organs has been realised, and 

 even now their functions in some cases have not been definitely 

 ascertained. Some are concerned with keeping the blood in proper 

 condition, and these are termed the lymphoid " glands." Although 

 perhaps it is better not to call these glands at all, since they do not 

 form a secretion, they are included here for convenience. The 

 others, true glands, by means of their internal secretions, which 

 contain one or more enzymes, termed hormones, have a far-reaching 

 effect on the metabolism of the body as a whole, or upon certain 

 parts. 



Spleen. 



This is a small dark red almost spherical body, situated in 

 the mesentery close to the anterior end of the rectum. Inside its 

 peritoneal covering is a connective tissue capsule, and within this 

 again a framework of connective tissue, in which is contained a large 

 number of closely packed cells of various kinds, forming the splenic 

 pulp. Some of the cells are red corpuscles and different varieties 

 of leucocytes. It is a lymphoid gland, generally considered to be a 

 centre in which leucocytes and probably red corpuscles are produced, 

 and it also serves as a station for the destruction of worn-out cor- 

 puscles and the removal of pigment and other waste matters from 

 the blood. Other small lymphoid glands, probably with similar 

 functions, are to be found. In ourselves one pair form the tonsils. 



Thymus Gland, 



A small reddish coloured oval body about 2 mm. in length is 

 to be found just behind the annulus tympanicus and beneath the 

 depressor muscle of the mandible, this is the thymus gland. It is 

 derived from the dorsal side of the gill clefts of the embryo, and histo- 

 logically appears much like a lymphoid gland. In the adult it forms 

 a centre for lymphocyte formation, but in the very young animal 

 it appears to play a considerable part in the processes of nutrition. 



Thyroid Gland. 



This is a small compact oval mass lying just outside the 

 anterior end of the posterior cornu of the hyoid plate. It consists 



