VERTEBRATES. 47 



these. They arc omnipresent in the body, and are to be found jnisliing their processes 

 into the spaces which separate the elements of the otlier tissues. In this way tlie 

 leucocytes play an important part in the absorption of food from the intestinal canal, 

 as well as in the swallowing up of the products of the waste of the tissues, and in the 

 removal of useless parts. The leucocytes so engaged are contained in the lymphatic 

 vessels, which do not constitute a system of closed tubes like the blood vessels, but 

 communicate with all the chinks and crevices of the tissues, and are further interrupted 

 here and there by the lymphatic glands — masses of a peculiar form of connective tissue 

 (adenoid tissue) which effect important changes in the lymjih traversing the irregular 

 spaces contained in them. We shall first consider the blood-vessels, as their constancy 

 affords valuable material for study to the morphologist, but it may be convenient to 

 introduce some general statements as to the development and structure of the vessels 

 of both categories. 



All the vessels are formed in that layer (splanehnoplure) of the mesoblast which 

 associates itself with the hypoblast to form the intestine. They are to be regarded as 

 spaces foi'med between the cells of the mesoblast by the collection of fluids, and, 

 consequently, if they are lined by cells which are disposed like epithelium ; these are, 

 nevertheless, of mesoblastic origin. The only other elements met with in the walls of 

 the vessels are of the nature of muscular and connective tissues. In certain tracts of 

 the vascular system the muscular fibres predominate, and they even acquire in the 

 heart a histological character, ajiproaching in differentiation that of the voluntary 

 musculature ; but in other tracts the walls are almost entirely formed of coimective 

 tissue, which may be largely of that kind known as elastic. 



Unlike the lymph vessels, the blood vessels form a system of closed tubes, so that 

 if a large vessel be cut across, and an easily-flowing fluid be injected into one end, it 

 will flow along the whole system without passing through the walls, and escape by 

 the other. The system is formed of central receiving and projielling chambers, con- 

 stituting the heart, from which arteries are given off to the various parts of the body. 

 These subdivide until extremely fine vessels, the capillaries, result, in which changes 

 in the gaseous and fluid constituents can take place, owing to the extreme delicacy of 

 their walls. From the variously-formed meshes resulting from the union of these 

 capillaries, the blood is drained by comparatively thin-walled veins, all of which con- 

 verge towai'ds the heart. From what has been said as to the function of the colored 

 blood cells, it is obvious that the vessels must have iuiportant relations to the respira- 

 tory organs. It is necessary to bear this in mind to understand the singular course 

 of development of the blood vessels in the higher forms, and here, as well as in the 

 case of the other systems, it is desirable to start with a comparatively primitive form. 

 Such a type is furnished by the bow-fin (Aini'a) the heart and great \essels of which 

 are represented in Figs. 42 and 43. 



As we have already seen, the heart is situated in a special cliMuiber of the body cav- 

 ity, — the pericardium (p. 11). The great veins converging towards it generally open 

 into a venous sinus, outside the pericardium, which in its turn has a large aperture into 

 the receiving chamber ])roper, situated on the dorsal aspect of the heart, and called the 

 atrium. The propelling chamber, or ventricle, lies on the ventral aspect, and projects 

 further caudad than the atrium ; it is somewhat iiointed behind, and in front is nar- 

 rowed into a conical projection, round which the ear-like ai)pendages of the atrium 

 (auricles) lap so as to appear on the ventral surface. The walls of the atrium are thin, 

 its cavity is subdivided by a sieve-like ])artition into a smaller left and larger right cham- 



