234 PRACTICAL HISTOLOGY. [xX. 



septum be placed in a weak solution of the blue, and then mounted in 

 ammonium picrate glycerine (p. 192). 



16. Circulation in the Tongue of Frog. Destroy the brain of a frog, and 

 after a time curarise it and fix it on its back on a frog-plate of cork with a hole 

 cut in it just in front of the head of the animal, the hole corresponding in 

 size to that of the tongue. The tongue is attached in front, and it can thus 

 readily be pulled out of the mouth so as to display its blood-vessels from the 

 under-surface. Pin out the tongue over the hole in the cork, when the under- 

 surface of the tongue will be uppermost. Fix the tongue under the microscope 

 and examine it (L). One can study the circulation in its vessels, and also 

 observe nerve-fibres in their normal condition. 



LESSON XX. 



THE LYMPHATIC SYSTEM SPLEEN TONSILS 

 THYMUS GLAND. 



THE lymphatic vessels have thin translucent walls, and in all 

 essential respects resemble veins in structure. There are three 

 coats in the larger vessels. The muscular fibres are abundant in 

 the middle coat, and the epithelium of the inner coat is in some 

 situations sinuous in outline. Valves are numerous in some 

 situations. The larger vessels spring from so-called lymph capil- 

 laries, which are usually wider than blood-vessels, and they unite 

 with each other and form an irregular plexus. They often present 

 dilations and constrictions, and consist of a single layer of endothe- 

 lium with sinuous outlines. They are without valves, and open into 

 the smallest regular lymphatic vessels. 



Lymphatic Glands. These vary much in shape, size, and colour, 

 but they are frequently oval or kidney-shaped. At one part is a 

 depression the hilum where the medullary part of the gland 

 comes to the surface, and where the blood-vessels enter the gland 

 and the efferent lymphatic vessel leaves it. There is usually only 

 one efferent vessel and several afferent lymphatics ; the latter per- 

 forate the capsule and enter the gland on its convex side. On 

 making a section of a gland, with the naked eye one can see that it 

 is divided into a cortical and a medullary part. The gland is 

 invested by a fibrous capsule, which in some animals contains 

 smooth muscular fibres. It consists of two layers, the outer of 

 coarser and the inner of finer connective tissue. It sends somewhat 

 flattened, large, usually unbranched, septa or trabeculse into the 

 cortex, thus dividing it into a series of compartments or alveoli 



