XI. THE COMPARATIVE ANATOMY OF THE CIRCULATORY 



SYSTEM 



A. GENERAL CONSIDERATIONS 



i. The parts of the circulatory system. — The circulatory system of vertebrates com- 

 prises two systems of branching tubes inclosing circulating fluids, the blood-vascular system 

 and the lymphatic system. The former is the larger and more conspicuous of the two and is 

 the one referred to when the expression circulatory system is employed without qualification. 



The blood-vascular system is a closed system, that is, it consists of a set of branching tubes, 

 the blood vessels, which are continuous with each other, unconnected with other systems (except 

 the lymphatic system), and in which the inclosed fluid travels in a circuit. The parts of the 

 blood-vascular system are the heart, the arteries, the veins, and the capillaries. The heart is a 

 contractile muscular organ situated in the median ventral region in the anterior part of the 

 body. It has essentially the form of an S-shaped tube subdivided into chambers which will 

 be named in connection with the dissection. The arteries are the vessels which leave the heart 

 and in which the contained fluid flows away from the heart. The veins are vessels in which the 

 contained fluid flows toward the heart. The student should particularly note that arteries and 

 veins cannot be defined on the basis of the kind of blood which they contain. The veins of 

 the vertebrate body fall into three classes: the systemic veins, which flow directly into the 

 right side of the heart; the pulmonary veins, which flow from the lungs into the left side of 

 the heart; and the portal veins or portal systems, in which the blood does not return directly 

 to the heart but passes into a system of capillaries from which it is re-collected into systemic 

 veins. The capillaries are the minute microscopic vessels which connect the ends of the arteries 

 with the beginnings of the veins and through which the circulation is completed. All the 

 tissues of the body are permeated with networks of capillaries through the walls of which the 

 gaseous and other exchange between the blood and the body cells takes place. 



The blood-vascular system incloses a fluid, the blood, which in vertebrates is colored red. 

 The blood consists of a colorless fluid, the plasma, in which float microscopic cells, the cor- 

 puscles. The latter are of two general kinds, red and white, the former giving the red color to 

 the blood. Study of the blood lies outside of the limits of this course. 



The lymphatic system is an open system, that is, it consists not only of branching tubes, 

 the lymph vessels, but of large spaces, the lymph sinuses; and it is further in communication 

 with the coelomic spaces of the body. Lymph sinuses occur beneath the skin (the student may 

 recall the large subcutaneous lymph sinuses in the frog), between the muscles, in the mesen- 

 teries, in the walls of the digestive tract, around the central nervous system, etc. From these 

 sinuses the fluid passes into more or less definite lymph vessels, which eventually open into 

 the veins of the blood-vascular system. In the lower vertebrates contractile lymph hearts are 

 placed in the course of the lymphatic vessels, to aid the flow, but these are absent in mammals. 

 The lymphatic system further differs from the blood-vascular system in that nodules of tissue, 

 the lymph glands, are placed in the path of the lymph vessels. The lymph glands consist of 

 a network of connective tissue in which are imbedded masses of cells, known as lymphocytes. 

 Lymphocytes are a variety of white blood corpuscles. The function of the lymph glands 

 appears to be to destroy foreign particles, bacteria, etc., and to add white blood corpuscles to 

 the circulation. The tonsils, the thymus, and the spleen belong to the category of lymph 

 glands. 



