50 NATURAL HISTORY OF VERTEBRATES. 



pair of gills in the adult Amia. They partly disappear with development, and are 

 partly converted into the great vessels proceeding from the heart ; this conversion 

 taking place in all in a fundamentally similar manner, although with numerous minor 

 modifications. Fig. 44 will serve to indicate the mode of the conversion of these in 

 the lizard. Here the atrium is completely subdivided into right and left auricles ; the 

 former receiving the blood from the body, the latter that from the lungs. Both open 

 into the ventricle, which is partially subdivided in such a way that only venous blood 

 is sent through the fifth pair of aortic arches (pulmonary arteries) into the lungs, while 

 mixed blood is sent to the body through the third and fourth pairs of arches. The 

 first and second pairs present in the embryo are unrepresented in the adult. 



It is easy to proceed now to the last term in the evolution of the vascular system, 

 that which obtains in the mammals. Here the ventricle is completely subdivided into 

 right and left chambers, the former of which propels the venous blood into the lungs 

 through the pulmonary artery, which is derived from the fifth aortic arch of the left 

 side only, while the latter propels the aerated blood received from the lungs by the 

 left auricle into the systemic aorta, the representative of the left fourth aortic arch of 

 the embryo. The two foremost pairs of aortic arches disappear without leaving any 

 trace, and the third is merely represented by parts of the carotid vessels destined for 

 the supply of the head. In the mammals it is obvious that the aerated arterial, and 

 the unaerated venous, blood, are thus kept unmixed, a condition which is reached also 

 in the birds, and almost perfectly in the crocodiles. 



The Lymph-Vasculae System. 

 "We have already touched upon the nature of the lymph, and on the roots of the 

 vessels in which it flows : a few words will suffice to indicate the disposition of the 

 vessels themselves. Like the veins, they are thin walled, and course towards the heart, 

 but the principal stem in which they terminate generally opens into one of the great 

 veins, not into the heart itself. As a rule, the larger lymph vessels are to be found 

 side by side with the blood vessels, although the smaller branches unite much more 

 freely with each other than is the case with the smaller arteries and veins. Many of 

 the lower vertebrates have lymph hearts, which assist in the propulsion of the contents 

 towards the heart ; but these do not occur in the mammals. The special way in which 

 the masses of adenoid tissue, referred to above, are interi)olated in the course of the 

 lymph vessels, is very different in different forms, but the lower vertebrates have much 

 less distinct and differentiated lymphatic glands than the higher forms. In the latter, 

 we distinguish as mesenteric glands those which are interpolated in the course of the 

 absorbent lymphatic vessels distributed to the intestinal walls. Although they can 

 hardly be said to be differentiated in the lower forms, yet certain masses of the 

 same sort of tissue have taken on special functions, apparently in connection with 

 the elaboration of the blood, so that they are often called blood-glands. The 

 most constant as well as the largest of these is the spleen, but the thyroid and 

 thymus, to which reference has already been made, evidently belong to the same 

 category. It is interesting to note that this sort of adenoid tissue may invade 

 organs of originally very different function, and convert them into lymphoid 

 organs. The thyroid and thymus furnish instances of this, while, in the Teleosts, the 

 foremost part of the kidney may become converted into a lymphoid organ, similar in 

 appearance to, and as bulky as, the spleen. In connection with the lymph vascular 

 system, it is proper to mention that the coelom and its various compartments are, in 



