IN GENERAL. 3 



function are probably in connexion with those of the lymphatics l . 

 This organ is situated in the neighbourhood of the stomach, and thus 

 more on the left side, at the upper part of the abdominal cavity; 

 it consists of a red, highly vascular tissue surrounded by a fibrous 

 external membrane, from which white productions as envelops of 

 the blood-vessels penetrate the internal soft substance and support 

 it like transverse joists. Peculiar vesicles, of microscopic size, 

 filled with a white pultaceous mass (MalpigJiian bodies), are attached 

 to these productions; the red pulpy substance contains many red- 

 brown granules 2 . 



We have already indicated that the blood of vertebrate animals 

 is red; hence the ancients, though incorrectly, ascribed blood to 

 these animals alone, and called the invertebrates bloodless animals. 

 The veins of the body convey the blood to the heart, from which it 

 is driven to the respiratory organs: that is, the heart of vertebrate 

 animals is venous. In invertebrate animals, on the contrary, the 

 heart is arterial, i. e. it receives those veins which return the blood 

 from the respiratory organs, and sends the blood to the vessels 

 which are distributed to the various parts of the body. In verte- 

 brate animals, an arterial heart also may be present, but never 

 unless at the same time a venous heart be found. The two hearts 

 in that case lie close together (the two ventricles in mammals and 

 birds), or they coalesce to form one cavity, as in most of the 

 reptiles. Thus, whenever the heart receives the veins of the body 

 alone, as in fishes, then there is only one auricle and one ventricle; 

 if, on the other hand, it receives the veins of the body (the vence 

 cavce), and the veins of the respiratory organs as well, then there 

 are two auricles (atria), but not on that account always two ven- 

 tricles. In those vertebrate animals in which the two ventricles 

 are united to form one cavity, not all the blood but only a part of 



1 It is asserted that the chyle acquires greater coagulability by admixture of fluid 

 contained in the lymphatics of the spleen. Compare P. W. LUND Physiologische 

 Residtate der Vivisectionen neuerer Zcit. Kopenhagen, 1825, 8vo, s. 78 83. 



2 The spleen appears to be absent in the Cyclostomes alone, although a gland 

 situated on each side of the superior orifice of the stomach is referred to it by MEYER 

 and others. Notwithstanding, however, the general presence of the organ in verte- 

 brate animals, it does not seem to be essential to life, but may be excised without 

 danger, as has occurred as well in animals as in man ; see the notes in HALLEB Ele- 

 ment. Physid. VI. p. 421. 



12 



