SECRETION. 325 



From the thyrnus various substances may be extracted, many of them 

 similar to those obtained from the spleen, e.g., xanthin, hypoxanthin, 

 and leucin, as well as certain proteids, especially nucleo-proteid (found 

 in all protoplasm), which on injection into the veins of an animal pro- 

 duces intra-vascular clotting. 



Function. Beard has recently concluded from some experiments on 

 the smooth skate that the important function of the thymus is the forma- 

 tion of the colorless corpuscles that the thymus, in fact, is the parent 

 source from which all the colorless corpuscles are derived. The first are 

 developed from the thymus cells, and from them all the others arise. 



Respecting tho thymus gland in the hybernating animals, in which it 

 exists throughout life, as each successive period of hybernation approaches, 

 the thymus greatly enlarges and becomes laden with fat, which accumulates 

 in it and in fat glands connected with it, in even larger proportions than 

 it does in the ordinary seats of adipose tissue. Hence it appears to 

 serve for tho storing up of materials which, being re-absorbed in inactivity 

 of the hybernating period, may maintain the respiration and the tem- 

 perature of the body in the reduced state to which they fall during that 

 time. It is also believed to be a source of the red blood-corpuscles, at 

 any rate in early life. 



The Pineal Gland. This gland, which is a small reddish body, is 

 placed beneath the back part of the corpus callosum, and rests upon the 

 corpora quadrigemina. 



Structure. It contains a central cavity lined with ciliated epithelium. 

 The gland substance proper is divisible into (1.) An outer cortical 

 layer, analogous in structure to the anterior lobe of the pituitary body; 

 and (2.) An inner central layer, wholly nervous. The cortical layer 

 consists of a number of close follicles, containing (a) cells of variable 

 shape, rounded, elongated, or stellate; (I) fusiform cells. There is also 

 present a gritty matter (acervulus cerebri) , consisting of round particles 

 aggregated into small masses. The central substance consists of white 

 and gray matter. The blood-vessels are small, and form a very delicate 

 capillary plexus. 



The pineal gland is a vestigial structure, being the atrophied third 

 eye which was situated in the median line. It is found in a better de- 

 veloped condition in certain lizards, though it is functionless. 



The Coccygeal and Carotid Glands. These so-called glands are 

 situated, the one in front of the tip of the coccyx, and the other at the 

 point of bifurcation of the common carotid artery on each side. They 

 are made up of a plexus of small arteries, are inclosed and supported by 

 a capsule of fibrous tissue, which contains connective-tissue corpuscles. 

 The blood-vessels are surrounded by one or more layers of cells like 

 secreting cells, which are said to be modified plasma cells of the connec- 

 tive tissue. The function of these bodies is unknown. 



