LEPUS CUNICULUS 325 



the main blood-vessels. It is relatively much larger in a young 

 animal and may then even partly cover the heart. When examined 

 in microscopic sections it is seen to be composed mainly of a lymphoid 

 tissue. It appears to be concerned with nutrition and metabolic 

 activities. 



The Thyroid Gland. This, the fiist ol; the ductless glands, is a 

 soft dark-coloured body situated at the anterior end of the trachea. 

 It is composed of two fairly large flat lateral lobes connected across 

 the mid- ventral line by a thin strand of its own tissue closely attached 

 to the trachea just below the thyroid cartilage. It is also subject 

 to considerable variation in size. It is found to consist of an inter- 

 lacing framework of connective tissue, which also forms a capsule 

 and binds together a number of spherical vesicles. The walls of 

 these vesicles are composed of a characteristic cubical glandular 

 epithelium, and the secretion of their cells is found to fill the vesicular 

 cavities with a readily coagulable fluid, which with its surrounding 

 cells gives to the section of this body a striking appearance. It has 

 already been noted that this secretion contains a protein rich in 

 iodine and apparently plays an important role in katabolic activities. 



Near each thyroid is a pair of small bodies termed the para- 

 thyroids, which are not composed of such typical glandular tissue. 

 In addition to these there are certain other small accessory para- 

 thyroids in different places, sometimes included in another body. 

 Their function is but little understood. 



The Supra-renal Bodies. In the rabbit these bodies are not 

 directly connected with the kidney as in man, but lie a short distance 

 away at the point where the renal vein of the same side flows into 

 the post-caval. They are conspicuous light yellow-coloured bodies 

 composed of two distinct varieties of tissue, termed the cortex on 

 the outside and the medulla within. 



The Pituitary Body. This body is lodged within the sella turcica 

 in the floor of the cranium, and is separated from the brain itself by 

 the interposition of a tough double fold of the dura mater save at 

 one point where it joins on to the infundibulum coming from the 

 floor of the third ventricle. When examined microscopically three 

 distinct regions can be distinguished. They are termed the pars 

 nervalis, the pars intermedium and the pars glandularis. The latter 

 is composed of typical glandular cells, and it arises in the embryo 

 from an outgrowth of the dorsal wall of the stomodreum or mouth 

 cavity which becomes completely cut off from the cavity in the 

 couise of development. It produces an important internal secretion 

 whose function has been discussed briefly when considering this 

 body in the frog. 



The ovaries and testes also must be regarded as ductless glands, 



