238 DUCTLESS GLANDS. 



discharged when the purposes of the economy so demand. The reser- 

 Yoir of the urine receives the whole of the secreted fluid ; in the gall- 

 bladder, on the other hand, only a part of the bile is collected. The 

 vesiculge seminales afford another example of these laterally appended 

 reservoirs. The ducts are constructed of a basement-membrane and 

 lining of epithelium, and in their smaller divisions there is nothing 

 more ; but in the larger branches and trunks a fibro-vascular layer is 

 added, as in the ordinary mucous membrane, with which many of them 

 are continuous, and with which they all agree in nature. A more or 

 less firm outer coat, composed of connective tissue, comes in many 

 cases, to surround the mucous lining, and between the two, or, at any 

 rate, outside the mucous coat, there is in some ducts a deposit of non- 

 striated muscular tissue. The epithelium is usually composed of 

 spheroidal or polyhedral cells at the commencement of the ducts, and 

 is columnar in the rest of their length, though sometimes flattened or 

 scaly, as in the mammary gland. 



DUCTLESS . OR VASCULAR GLANDS. 



There are certain bodies which have received the name of glands on 

 account of their resemblance in general appearance and structure to 

 the ordinary secreting organs. They differ, however, from the latter in 

 the fact of their possessing no duets for the discharge of secretion ; 

 so that their elaborated products must be conveyed into the blood by 

 lymphatic or sanguiferous vessels, with both of which they are for the 

 most part abundantly provided. The bodies in question have been 

 termed " ductless " for this obvious anatomical reason : and " vascular,"^ 

 on certain physiological or theoretic grounds, as they are supposed to 

 effect some change in the blood which is transmitted through them. 



To this class belong the following bodies : — the spleen, the thyroid 

 body, thymus gland, suprarenal capsules, pituitary body, the solitar}^ 

 closed follicles of the intestines, the Peyerian glands, the follicular 

 glands at the root of the tongue, and also the lymphatic glands. 



The purposes fulfilled by certain of the organs enumerated are still 

 involved in great obscurity, but the majority have apparently the same 

 fundamental constitution. They are essentially made up of corpuscles 

 having the character of lymph- or pale blood-corjDuscles, included and 

 supported by retiform connective tissue, traversed tliroughout by snn- 

 guiferous capillaries, and provided with numerous and large lymphatic 

 vessels : their constituent substance, in fact, agrees in structure with 

 what has already been described as lymphoid tissue. This is in some 

 cases formed into small rounded l)odies or pellets, which may be either 

 closely grouped together to form the lobules of a gland-like organ 

 (thymus) or distributed in other tissues (follicular glands of mucous 

 membrane, Malpighian corpuscles of the spleen) : in other cases the 

 lymphoid substance is in large masses, which may take on a more or 

 less reticular arrangement, with lymph-sinuses occupying the inter- 

 stices of the network (lymphatic glands). 



Some of the organs in question, for example, the thyroid gland, differ 

 altogether from the rest, except in the absence of ducts. The account 

 of these and of the specialities of the " lymphoid" organs is reserved for 

 the part of this work devoted to special anatomy. 



