DEVELOPMENT OF THE VASCULAR SYSTEM. 511 



pervading the entire gland, all of whose rounded spaces, ^ to -J g 

 of an inch wide, communicate freely. 2. The grayish-white alka- 

 line pulp filling them, agrees, in almost all respects, with that in the 

 follicles of Peyer. (Fig. 360). 3. Vessels also enter these spaces 

 and form a fine vascular network as in the follicles just mentioned. 

 (Fig. 341, A.) 



The lymphatic glands are, therefore, not mere convolutions of 

 the lymphatic vessels; but distinct and independent structures, in 

 the areolae (alveoli, Kblliker) of which the vasa afferentia terminate, 

 and the efferentia commence. The cells contained in the pulp 

 seem identical with the lymph- (cytoid) corpuscles of the blood ; and 

 a portion of the latter may, perhaps, be developed here. The vasa 

 afferentia lose their muscular layer as they enter the gland, and 

 enter the alveoli with only a layer of areolar tissue and an epithe- 

 lium. Nervous filaments composed of fine fibres enter the glands 

 with the vessels, and disappear in their interior. 



The most common degeneration of the lymphatic glands is pro- 

 duced by the extravasation of blood into the alveoli, leading to 

 deposits of quite dark pigmentary matter (as in the bronchial 

 glands). The sheaths, or internal septa, are also liable to become 

 thickened, and fatty deposits to occur in the bloodvessels. Hyper- 

 trophy of all their elements may also occur, and tuberculous and 

 cancerous deposits. 



Functions of the Vascular System. 



For the precise role of each part of the blood-vascular system in 

 securing the circulation of the blood, and for the action of the 

 lymphatics, reference must be had to the works on physiology. 

 The blood and lymph (and chyle) have already been described at 

 length (p. 147-79). 



Development of the Vascular System. 



1. The rudiments of the heart, arteries, and veins are alike solid 

 tracts of cells, of greater or less thickness ; which, by a liquefaction 

 internally, and a transformation of the central cells into blood-cor- 

 puscles, become cavities and continuous passages for the blood. 

 The heart manifests contractions while still in the form of a cellular 

 tube ; and subsequently the cells occupying the walls elongate into 

 the muscular and other fibres. At the same time, the vessels be- 

 come thicker, and increase in circumference, at first by an augmen- 

 tation of the number of the cells; afterwards chiefly, if not solely, 



