208 THE MICROSCOPIST. 



glands and tonsils, the solitary and agmiuated glands of 

 the intestine (Peyer's patches), the thymus, and the spleen 

 have a similar structure, and are called lymphoid organs. 



In the thoracic duct the epithelium is inclosed in several 

 layers of fibrous membrane. The latter contains trans- 

 verse muscular fibres. The heart, although an involuntary 

 muscular organ, has striated muscular fibres. These fibres 

 are not, like other striped muscles, collected into bundles, 

 but are reticular. The heart, like other organs, is supplied 

 with lymphatics and bloodvessels. The cardiac plexus- of 

 nerves consists of branches from the vagus and sympa- 

 thetic. Numerous microscopic nervous ganglia also occur, 

 especially near the transverse groove and septum of the 

 ventricles. It is thought that these are the chief centres 

 of energy, so that the heart pulsates after its removal from 

 the body. It has also been shown recently that the sym 

 pathetic and vagus filaments are in antagonism, so that 

 stimulation of the vagus interrupts the motor influence of 

 the sympathetic, and may bring the heart to a standstill 

 in a condition of diastole. 



The structure of bloodvessels has been described under 

 the head of vascular tissue. No special boundary exists 

 between capillaries and the arteries and veins. The ar- 

 rangement of the capillaries, however, is various, and 

 often so characteristic that a practiced eye can generally 

 recognize an organ or tissue from its injected capillaries. 

 (Plate XXII, Figs. 163 to 168.) For methods of inject- 

 ing, see page 64. Capillaries form either longitudinal or 

 rounded meshes. The muscular network, etc. , is extended, 

 while fat-cells, the alveoli of the lungs, lobules of liver, 

 capillary loops of papillae in skin and mucous membranes, 

 outlets of follicles, etc., present a more or less circular in- 

 terlacement. The capillary tube lies external to the ele- 

 mentary structure, and never penetrates its interior. 



2. Secretive Organs. True secretions serve important 

 offices in the organism : as the materials of reproduction ; 



