THE THYMUS. 



153 



exhilarating ideas diminished it. The causes of these changes are referable not 

 only to changes in the amount of blood in the spleen, but also to the greater or less 

 degree of contraction of its muscular tissue. And it would appear that, like the 

 small arteries, the muscular tissue of the spleen is in a state of tonic contraction. 

 The size of the spleen may be influenced reflexly. Thus, Tarchanoff found that 

 stimulation of the central end of the vagus, when the splanchnics were intact, 

 -caused contraction of the spleen, while stimulation of the central end of the 

 .sciatic also caused contraction, but to a less degree. It is quite certain that all the 

 phenomena are not due to the action of vaso-motor nerves on the splenic blood- 

 vessels. There is a certain amount of independent action of the muscular fibres of 

 the organ, and it is not improbable that the innervation of the spleen is similar to 

 the innervation of arteries, and that it has a motor centre in the cord capable of 

 being influenced reflexly by afferent nerves, while it also sends out efferent im- 

 pulses.] 



[Stimulation of (1) the central end of a sensory nerve ; (2) of the peripheral ends 

 of both splanchnics ; (3) of the peripheral ends of both vagi, causes contraction of 

 the spleen. But even after section of the splanchnics and vagi, stimulation of a 

 sensory nerve still causes contraction, so that there must be some other channel as 

 yet unknown {Roy). Bochefontaine found that electrical stimulation of certain 

 parts of the cortex cerebri produced contraction of the spleen.] Sensory nerves 

 seem to occur only in the peritoneum covering the spleen. 



Pressure on the splenic vein causes enlargement of the spleen, hence, increased pressure in 

 this vein (congestion of the portal vein, cessation of heemorrhoidal and menstrual discharges) 

 also causes its enlargement. With regard to the action of "splenic reagents," such as quinine, 

 on the contraction of the spleen, Binz is of opinion that this drug retards the formation of the 

 colourless blood-corpuscles, so that its chief function is interfered with and the organ becomes 

 less vascular. It is not definitely decided, however, whether it is contraction or dilatation of 

 the spleen that alters the proportion of red and white corpuscles in the blood. 



Splenic Tumours. The increase in size of the spleen in various diseases early attracted the 

 attention of physicians. The healthy spleen undergoes several variations in volume during 

 the course of a day, corresponding with the varying activity of the digestive organs. In this 

 rrespect the spleen resembles the arteries. In many fevers the spleen becomes greatly enlarged, 



probably due to paralysis of its nerves. It is greatly 

 increased in intermittent fever or ague, and often 

 during the course of typhus. When it becomes ab- 

 normally enlarged, and remains so after repeated 

 attacks of the ague, it is greatly hypertrophied, and 

 constitutes "ague cake." 

 In cases of splenic leuktemia 

 it is greatly enlarged, and 

 at the same time there is a 

 great increase in the number 

 of colourless corpuscles in 

 the blood and also a decrease 

 of the coloured ones ( 10). 



II. The Thymus. During 

 f cetal life this gland is largely 

 developed, and it increases 

 during the first two or three 

 years of life, remaining sta- 

 p- -t 05 tionary until the tenth or 



Section of the thymus gland of a cat, with ^ JJ^JS'^ Undergo E1 ? me "^ of the ^ m ^ s 



one complete lobule with a cortical 



part a, and a centre, 



wiwi a ^i fatty degeneration. [Thede- l xow /; 

 b. . a, lymphoid ' at f on w ins L at the corpuscles 



generation begins at the 



( x 300). a, lymph- 

 corpuscles ; b, con- 

 tissue ; c, blood-vessels injected ; d, con- gE^gftf each" lobule and g 11 * cor P uscle of 

 nective-tissue. progresses inwards (His).] HassalL 



Structure. "It consists of an aggregation of lymph-follicles (resembling the glands of 

 Peyer) or masses of adenoid tissue held together by a framework of connective-tissue which 

 contains blood-vessels, lymphatics, and a few nerves (fig. 125). The framework of connective- 

 tissue gives off septa which divide the gland into lobes, these being further subdivided by finer 



