482 



GLAND. 



nrity which, with the aid of Comparative Ana- 

 tomy, has been so satisfactorily demonstrated 

 in the development of the nervous and osseous 

 systems, is equally evinced in the glands ; for 

 whatever diversities may be presented in their 

 form and appearances whatever varieties may 

 be remarked in the internal disposition of their 

 component parts, as in contrasting a simple 

 follicle with ;\ conglomerate gland, or the tubu- 

 lar biliferous organs of insects with the appa- 

 rently solid liver of the Mammalia; whether, in 

 short, they appear solid, cellular, or tubular, 

 every glandular organ is nothing else than a 

 modification of a simple closed sac. This im- 

 portant truth is distinctly announced by Meckel 

 in the following passage. " The most simple 

 mucous glands, which are only simple sacs, 

 present the type of the glandular formation. If 

 we picture to ourselves this sac as being pro- 

 longed and ramified, and interlacing its branches 

 between those of the bloodvessels, we shall at 

 length arrive at the most compound gland, 

 without there ever being a direct communica- 

 tion between the bloodvessels and the excretory 

 ducts."* We have here briefly but clearly ex- 

 pressed the great principle, in obedience to 

 which the various glands are developed ; but 

 that which Meckel only figuratively expressed, 

 has since been realized in all its bearings and 

 intricate details, by the extensive and laborious 

 researches of several distinguished anatomists, 

 and especially by Professor Miiller of Berlin. 



As it would be in vain to attempt to demon- 

 strate the essential characters of the glandular 

 formation, and to prove the uniformity which 

 pervades the whole system, by selecting, as has 

 been generally done, the most intricate organs ; 

 I propose, in the first place, to describe the 

 most simple form of gland, and, seizing this as 

 a clue, to trace its gradual development through- 

 out the whole series of glandular organs, so as 

 to convey a general, but, it is hoped, compre- 

 hensive account of this interesting branch of 

 anatomy. 



With this object in view, the simple follicles 

 of the skin and mucous membrane may be ad- 

 vantageously selected ; because by tracing the 

 successive development of these bodies, the 

 gradual transformation of a simple sac into a 

 tube, a ramified canal, and even a conglome- 

 rate gland, may be very distinctly demonstrated. 

 In fishes, whose aquatic mode of life renders 

 an abundant defensive secretion necessary, the 

 cutaneous follicles are more developed than in 

 other animals, and constitute tubes or canals, 

 which being carefully examined are found to 

 end in ccecal extremities. A similar formation 

 is seen in the bulbus glandulosus of most birds, 

 where the mucous crypts are prolonged into 

 short tubuli (Jig. 209) ; whilst in the Ostrich 

 (Struthio camelus) the follicles present an ap- 

 pearance of cells. ( Fig. 209, 4). In some 

 Amphibia, Salamandra maculata for example, 

 the glands of the external integument being 

 very much developed, it is seen that each of 



* Man. d'An. i. p. 515. Declare! has a similar 

 comparison : " it is true that a gland, like a folli- 

 cle, consists of a canal closed at the extremity." 

 Anat. Gen. p. 424. 



Fig. 200. 



a. Conglomerate folli- 

 cula r gland , S(r*Aio rhea ; 

 c. same, Meleagris ; d. 

 same, Aiuer ;* the upper 

 drawing shows the cylin- 

 drical follicles in a young 

 falcon. 



Fig. 210. 



those bodies is com- 

 posed of a small flask- 

 shaped pouch of the 

 skin, which at one ex- 

 tremity becoming en- 

 larged into a base, 

 there terminates in a 

 blindsac ; whilst at the 

 other end being con- 

 tracted, it opens by a 

 short neck on the ex- 

 ternal surface. (See 

 fig. 210.) A micro- 

 scopical view of the 

 ccecal canals in the 

 simpler glands is ap- 

 pended. (See^g. 2 11.) 



Fig.Zll. 



Flask-shaped cutaneout The simple sacs and 

 follicle or gland magni- t^}^ j ust described are 



Jied HO.t very often C0 nected toge- 



ther, giving rise to aggregate or compound folli- 

 cles, the arrangement and degree of complexity 

 of which are very various. In some instances 

 these sacs unite so as to form a gland with a single 

 orifice or excretory duct, of which the Meibo- 

 mian glands of the eye-lids are an example ; or 

 again, the aggregate follicles may themselves be 

 joined together with various degrees of compli- 

 cation, in the form of a cluster, from which 

 several excretory ducts proceed, as in the cu- 

 runcula lachrymalis, the labial, buccal, and 

 other mucous glands of the mouth, the tonsils, 

 &c. In all these instances there is nothing but 

 an evolution of the original sac, so that in the 

 same manner as this is formed from the mucous 

 membrane or skin, are the tubes and canals 

 prolonged from the third pouch, by which con- 

 trivance the surface subservient to secretion is, 

 within a given space, greatly increased. The 

 conversion of a sac or tube into a granular mass 

 is also in this manner rendered very apparent ; 

 and thus it is easy to understand how a rami- 

 fied canal may produce the apparently solid 



* Home, Lect. on Comp. An. ii. tab. 46. 

 t Berres. 



