212 BLOOD AND BLOOD-FORMING ORGANS. 



leave at this point. In the cortical substance occur changes which 

 result in the formation of structures closely resembling the cortical 

 nodules of lymph-glands. 



Until recently, little was known of the significance of this organ. 

 A careful study revealed a similarity between certain cellular ele- 

 ments of the thymus and the constituents of the blood-forming 

 organs, a similarity still more striking from the presence of 

 nucleated red blood-cells in the thymus. Logically, then, the 

 embryonal thymus is to be regarded as one of the blood-forming 

 organs (Schaffer, 93, I). 



During embryonic life from the fourth month on and for some 

 time after birth, there are found in the thymus peculiar epithelial 

 bodies, known as the corpuscles of Hassal. They are spheric struc- 

 tures, about o. i mm. in diameter, whose periphery shows a con- 

 centric arrangement of the epithelial cells. In their central portions 

 are found a few nuclear and cellular fragments. These bodies 

 occur only in the thymus gland. They are remnants of the primary 

 epithelial, glandular structure of the thymus, and are formed by an 

 ingrowth of mesoderm which breaks down the epithelium into small 

 irregular masses, mechanically compressed by the proliferating 

 mesoderm. 



The thymus gland has a relatively rich blood supply. Arterial 

 branches enter the lobules usually near the medullary cords and form 

 capillary networks at the boundaries of the medullary and cortical 

 portions ; from this anastomosing capillaries radiate to the periphery 

 of the lobules, joining to form a relatively dense capillary network 

 under the connective-tissue covering. The veins arise from this cap- 

 illary network and are situated mostly in the interlobular connec- 

 tive tissue. Certain of the veins are in the medullary portions of 

 the lobules, where they accompany the arteries (Kolliker, v. Ebner). 



The lymph-vessels are in the interlobular connective tissue in 

 close apposition with the adenoid tissue. 



Nerve-fibers accompanying the blood-vessels have been ob- 

 served. 



II. THE CIRCULATORY SYSTEM. 



THE walls of the blood-vessels vary in structure in the different 

 divisions of the vascular system. All the vessels, including the 

 heart, possess an inner endothelial lining. In addition to this, the 

 larger vessels are provided with other layers, which consist, on the 

 one hand, of connective and elastic tissue and, on the other, of non- 

 striated muscle-fibers. The vessels are also richly supplied with 

 nerves, that form plexuses in which ganglion cells are sometimes 

 found, and in the larger vessels the outer layer is honeycombed by 

 nutrient blood-vessels, called vasa vasorum. In the heart, the mus- 

 cular tissue is especially well developed. According to the structure 



