CHAP. VIIL] ORGANS OF RESPIRATION AND SECRETION. 237 



to the urethra the fibres assume a circular course, and so form a 

 somewhat dense mass called the sphincter of the bladder. 



External to the muscular coat is the fibrous investment of the 

 bladder. 



The passage by which the urine passes from the bladder to the 

 exterior is named the urethra. 



The urine is expelled by contraction of the muscular coat and 

 relaxation of the sphincter, and is aided by contraction of the 

 abdominal muscles and diaphragm and by a small muscle called the 

 accelerator urince (Fig. 115, "). 



13. The SUPRARENAL CAPSULES are two small roundish, somewhat 

 flattened bodies (Fig. 105, sr), situated one in the vicinity of the inner 

 part of the anterior end of each kidney, being attached there by 

 areolar tissue. Each consists of an outer cortical part, of a firm 

 consistency and yellow colour, and an inner softer and darker 

 medullary portion. It has a fibrous investing membrane intimately 

 connected with the cortical substance, which is made up of delicate 

 filamentous tissue with interspaces filled with granular matter, 

 nucleated cells and oil globules. These organs are richly supplied with 

 nerves, but they have no duct, and their function is quite unknown. 



14. The THYROID BODY or gland is another ductless structure of 

 unknown function. It is very vascular, of a soft consistency and 

 reddish colour, and lies (Fig. 112, tg) beside the trachea just below 

 the larynx. It consists of two lateral parts, each of about the size 

 of a small bean. There is no median connecting portion, so that 

 there should rather be said to be two thyroid glands than one. It 

 is composed of a number of minute, closed vesicles, which contain 

 a yellowish, glairy fluid, and are connected by areolar tissue ; a thin 

 but dense layer of which is the external investment. The thyroid 

 body is supposed by some physiologists to regulate the supply of 

 blood to the brain, and to prevent undue cerebral pressure by 

 acting as a collateral blood reservoir. 



15. The TIIYMUS is a structure which is of very large size during 

 immaturity, but becomes smaller in the adult cat. Like the thyroid 

 it is a ductless structure of unknown function. It lies on the dorsal 

 side of the sternum on the ventral aspect of the great vessels, and 

 extends j also far up the neck on the ventral surface of the trachea, 

 as represented in Fig, 112. It is of a greyish or pinkish colour, and 

 is soft and pulpy in consistency. It contains a central cavity, 

 around which are arranged a number of lobes and lobules made up 

 of delicate cells. A milky fluid is found within it, containing many 

 nuclei and small nucleated cells. 



16. Other DUCTLESS GLANDS are the closed follicles which make 

 up the " Peyer's patches," and " solitary glands " of similar nature 

 have been already mentioned as being found in the intestine. An- 

 other very small structure of a similar nature is called the " pitui- 

 tary body." It will be hereafter noticed in describing the brain, to 

 the under surface of which it is attached, lying within the sella 

 turcica or the dorsum of the basi-sphenoid bone (Fig. 128). Yet 



