IIEAET AND AETEEIES OF YEETEBEATA. 577 



important functions of the blood-corpuscles in the economy of the 

 Vertebrate., as carriers of gases, their number, size, and the surface 

 which they thereby represent, is of the greatest importance. In 

 the higher divisions the relative quantity of blood varies but little, 

 and there is not any great difference between the relative volumes 

 of the plasma and blood-corpuscles. With regard, however, to the 

 distribution of the whole substance of blood-corpuscles in larger or 

 smaller form-elements, there is a great difference between the cold 

 and warm-blooded divisions, and, again, between the Reptilia and 

 Amphibia ; from this point of view, the latter occupy a much lower 

 position. 



Welckek, H., Zeitschr. f. rationelle Med. XX. p. 290 ; und Arch. f. Mikrosk. YITI. 



Heart and Arterial System. 

 § 431. 



At a certain stage the heart of all Cranio ta is formed of a single 

 tube. As it gradually gets longer than the space set apart for it, it 

 is arranged in an S-shaped loop, and so takes on the form which the 

 heart has later on. As it changes its form, it is divided into two 

 parts. The hinder one receives the blood and sends it to the 

 anterior one, which conducts it into the vascular arches, and so into 

 an arterial trunk which runs along the axial skeleton, from which it 

 is distributed in the body. The first portion of the heart is called 

 the auricle, the second the ventricle. A special cavity, which is 

 similar to the heart when it is first formed, encloses the auricle and 

 ventricle; this is the pericardial cavity, and its walls form the 

 pericardium. 



We find this simple condition of the heart in Fishes. A 

 ventricle and an auricle form the two chief portions. The 

 latter receives blood from a sinus venosus, which is placed just 

 behind it, and is only partly outside the pericardium. As a rule it 

 has diverticula on either side, which grow out towards the chamber 

 in front of them (auricula)). The walls of the heart vary in character 

 according to their function. The wall of the auricle is provided 

 with a very thin muscular layer which projects inwards in a retiform 

 manner. Its only function is to drive the blood into the ventricle. 

 The ventricular wall has a more important function, and is conse- 

 quently provided with a powerful network of muscular bands. This 

 projects inwards, and so diminishes the size of the proper cavity of 

 the ventricle. But this is made up for by the connection between 

 the spaces in the network of the ventricular wall and the cavity of 

 the ventricle itself. There are two membranous valves at the 

 ostium atrio-ventriculare (Fig. 824, o), which prevent the blood 

 from returning to the auricle. There are generally as many as 

 three such pouched valves at the ostium arteriosum of the ventricle. 



2 r 



