l68 HISTOLOGY 



many small blood cells that pass down the duct into the blood stream. 

 They have no granules in their cytoplasm and probably develop into 

 both kinds of corpuscles (see lobster's blood) in the different parts of 

 the channel's system. Small striated muscle fibers are sometimes seen 

 in the walls of the duct (Fig. 149). 



A group of several other glands of somewhat doubtful meaning have 

 been described as blood-forming glands in the mollusks. They are 

 found in the neighborhood of the heart and are of various degrees of 

 concentration. One is rather widely distributed through the upper 

 mantle tissue around the heart region of Unio and other lamellibranch 

 mollusks. Another is found in a more compact form at the base of the 

 gill in Loligo and Octopus. The weight of somewhat unsatisfactory 

 evidence has tended to show that these organs are excretory, while by 

 some they are thought to be blood-making in function. We have treated 

 them under the heading of excretion. 



An enormous gap exists between the one, simply organized kind of 

 blood gland found in the crayfish or the Echinoderms, and the great 

 variety, number, and complexity of blood glands found in the mammals. 

 In man these glands are used to destroy blood as well as to form several 

 kinds of blood cells and perhaps to perform other functions as well. 



The fundamental idea, that these glands are highly differentiated 

 regions of the blood-channel wall, is difficult to maintain and, most prob- 

 ably, must have added to it a conception of a part of these organs as differ- 

 entiated areas of mesenchymal tissues which, while in close functional 

 relation to the blood vessels, are not morphologically a part of their 

 walls. The exact study of the blood and the blood glands of mam- 

 mals is, perhaps, the most difficult in histology as well as the one 

 which will give the richest results, if such comparisons may be per- 

 mitted. Its chief difficulty and interest lie in the fact that its cellular 

 elements are movable during the same time that they are changing, which 

 makes their history very hard to put together by studying dead sections. 

 The structures cannot be studied in situ during life. 



We shall study a smaller lymph node, or nodule as an example of 

 one of the more primitive blood glands in man. Such a blood center 

 may be found in many positions in the body and appears macroscopically 

 as a small lump of tissue with several blood and lymph vessels entering 

 or leaving it. 



A gland of this kind begins as a differentiation of the mesenchyme 

 in the neighborhood of some blood and lymph vessels. Branches of 

 both kinds of vessels enter the mass. The blood vessels enter as arteries 

 and, after forming a capillary circulation in the pulp, return as veins 

 from the same point. This point is called the hilum. The lymph chan- 

 nels also enter into the pulp and form a wide-meshed plexus in its sub- 



