720 THE LYMPHATICS. 



masses are surrounded by the lymph sinuses. In the centre of the meshes of 

 the adenoid tissue is a dark portion filled with cells showing karyokinetic division 

 — this is a germ centre, which is more particularly the seat of the formation of 

 leucocytes which pass into the lymph sinuses, and thence into the vasa efferentia. 

 The sinuses are lined by a single layer of flat endothehal cells, a similar layer 

 covering the surface of the round masses in the cortex, the strands in the 

 medulla, and the surface of the trabeculse — these cells being even found in the 

 bands of reticular connective tissue passing from the trabecule. 



Lymphatic glands are richly supplied with blood-vessels, either entering the 

 hilum or distributed over the surface. Those on the surface ramify on the 

 capsule, and penetrate the gland by running along the centres of the trabeculae. 

 The large vessel entering the hilum divides into branches, surrounded by con- 

 nective tissue, and these also run along the trabeculae. From these minute 

 branches spring, which pass through the lymph sinuses, and are ultimately dis- 

 tributed to the round masses in the cortex and to the strands in the medulla, 

 ending in a very fine-meshed capillary network. The veins pass out of the gland 

 at the hilum. Nerves also penetrate lymphatic glands, and medullated and non- 

 medullated fibres have been traced, but their mode of ending is unknown. 



It is important to observe that adenoid tissue containing leucocytes is not 

 limited to lymphatic glands, but is found in many mucous membranes some- 

 times diffused in a stratum, and sometimes in sharply defined masses ; small 

 nodules or follicles are also found in mucous membranes, containing germ 

 centres, and resembling the round masses in a lymphatic gland. These are 

 seen in the solitary and agminated glands of Peyer in the intestinal canal ; 

 they vary much in different species of animals, and even in individuals of the 

 same species ; and they differ from ordinary lymphatic glands, chiefly in the less 

 intimate connection existing between them and lymphatic vessels. The leucocytes 

 originating in them probably do not enter the lymphatic system directly, but 

 wander through the epithelium covering the surface of the mucous membrane. 

 Amphibia have no lymphatic glands, but, as may be seen in the Frog, there are 

 large lymphatic spaces beneath the skin, which are traversed by very delicate 

 bands of connective tissue. 



Preparation of the Lymphatic Vessels. — The lymphatic networks can only be studied 

 after having been filled with mercury by means of injection; but as this operation is not 

 usually practised by the pupils for whom this book is written, the mode of performing it will 

 only be traced in a few words. 



The apparatus in use consists of a glass tube continued by a flexible one, which carries at 

 its inferior extremity an iron tap and a fine cannula, also of iron, or (better) glass. To apply 

 this apparatus, the tube ought to be suspended and then filled with mercury ; the cannula is then 

 seized by the right hand, keeping it parallel to the membrane we wish to inject, and burying 

 it in the most superficial layer of that membrane. The extremity of the cannula is thu& 

 introduced into th*: midst of the meshes of the lymphatic network, and necessarily wounds 

 some of the capillaries which compose it. In opening the tap, the mercury is allowed to flow 

 into the capillaries by the solutions of continuity they present, and fills them in the most per- 

 fect manner. The lymphatic plexuses being always superposed on the capillary blood-vessels, 

 one is always certain of injecting them only, in taking the precaution to penetrate the membrane 

 as superficially as possible. If the point of the cannula enters too deeply, the mercury will 

 pass into the veins, and the operation will be unsuccessful, and must be commenced again. 



To study the branches and lymphatic trunks, it will sufiice to inflate them from their origin 

 towards their termination. This procedure, properly conducted — and it was almost exclusively 

 the only one adopted by the older anatomists — gives the most satisfactory results, and is even 

 suificient to demonstrate the texture of the glands. Or the trunks may be filled, from their 

 termination to their origin, by some solidifiable substance. 



