336 THE DUCTLESS GLANDS [CH. XXV. 



It has been shown by the oncometer (see p. 309) that the spleen 

 undergoes rhythmical contractions and dilatations, due to the con- 

 traction and relaxation of the muscular tissue in its capsule and 

 trabeculae. A tracing also shows waves due to the rhythmical alter- 

 ations of the general blood-pressure. Fig. 278 is a typical tracing 

 obtained by Schafer's air oncometer from a dog's spleen. 



It shows, first, the large waves occurring about once a minute, 

 due to the splenic systole and diastole; secondly, smaller waves on 

 this, due to the effect of respiration on the blood-pressure ; and on 

 these, smaller waves still, corresponding with the individual heart- 

 beats. The large waves due to the splenic contractility still go on 

 after the division of all the splenic nerves. These nerve-fibres leave 

 the spinal cord in numerous thoracic anterior roots ; they have cell- 

 stations in the sympathetic chain (Schafer) or semi-lunar ganglia 

 (Langley). 



Haemolymph Glands. 



The existence of glands which partake of the nature both of the 

 spleen and of lymphatic glands, has long been known. They have 

 been recently more fully investigated by T. Lewis. He finds them in 

 most mammals, and they can be readily distinguished from ordinary 

 lymphatic glands by their red colour. He divides them into (1) hcemal 

 glands, which are characterised by the fact that the sinuses contain 

 blood only; the spleen is in fact a large haemal gland; and (2) 

 hcemal lymphatic glands, in which the sinuses are filled by a mixture 

 of blood and lymph. 



The Thymus. 



This gland attains its greatest size soon after birth, and after the 

 second year it gradually diminishes, until in adult life hardly a 

 vestige remains; it is then replaced by adipose and connective 

 tissue. This, at any rate, has been the general belief until the last 

 few years. Some recent observations, however, appear to show that 

 the thymus persists longer, and may grow until puberty ; and that 

 some true thymus tissue may persist throughout life. 



The gland is surrounded by a fibrous capsule, which sends in 

 processes, forming trabeculas, that divide the gland into lobes, and 

 carry the blood- and lymph-vessels. The large trabeculse branch into 

 small ones, which divide the lobes into lobules. The lobules are 

 further subdivided into follicles by fine connective tissue. A follicle 

 is polyhedral in shape, and consists of cortical and medullary por- 

 tions, both of which are composed of adenoid or lymphoid tissue, 

 but in the medullary portion the matrix is coarser, and is not so 

 filled up with lymphoid corpuscles as in the cortex. Scattered in 

 the lymphoid tissue of the medulla are the concentric corpuscles of 



