4 i 6 THE DUCTLESS GLANDS. 



this modified form of secretion takes place, are usually 

 described as vascular glands, or glands without ducts, and 

 include the spleen, the thymus and thyroid glands, the 

 supra-renal capsules, and, according to (Esterlin and 

 Ecker, and Gull, the pineal gland and pituitary body; 

 possibly, also the tonsils. 



Peyer's glands of the intestine, and lymph-glands in 

 general, also closely resemble them ; indeed, both in 

 structure and function, the vascular glands bear a close 

 relation, on the one hand, to the true secreting glands, and 

 on the other, to the lymphatic glands. 



The evidence in favour of the view that these organs 

 exercise a function analagous to that of secreting glands, 

 has been chiefly obtained from investigations into 

 their structure, which have shown that all the glands 

 without ducts contain the same essential structures as the 

 secreting glands, except the ducts. They are mainly com- 

 posed of vesicles, or sacculi, either simple and closed, as in 

 the thyroid (Simon), spleen, and supra-renal capsules 

 (Ecker), or variously branched, and with the cavities of 

 the several branches communicating in and by common 

 canals, as in the thymus (Simon). These vesicles, like the 

 acini of secreting glands, are formed of a delicate homo- 

 geneous membrane, are surrounded with and often traversed 

 by a vascular plexus, and are filled with finely molecular 

 albuminous fluid, suspended in which are either granules 

 of fat, or cytoblasts or nuclei, or nucleated cells, or a mix- 

 ture of all these. 



These general resemblances in structure between the 

 vascular glands and the true glands lead to the supposi- 

 tion that both sets of organs pursue, up to a certain 

 point, a similar course in the discharge of their functions. 

 It is assumed that certain principles in an inferior state of 

 organization are effused from the vessels into the sacculi, 

 and gradually develop into nuclei or cytoblasts, which may 

 be further developed into cells ; that in the growth of these 



