126 HISTOLOGY. 



posterior cardinal veins; the musculature of the heart grows into the 

 cavity of the ventricle in plates and columns covered with endothelium 

 (Fig. 160), thus producing a net of vascular spaces or sinusoids. Although 

 the sinusoidal circulation persists in these organs in lower vertebrates, 

 such as the frog, it is not retained in man. The sinusoids of the heart are 

 reduced to shallow spaces between the columns of muscle seen on its inner 

 surface, and those of the mesonephros disappear with the transformation 

 of that organ into the epididymis and epoophoron in the male and female 

 respectively. Thick walled subdivisions which may occur in the course 

 of a vessel are not sinusoids. The latter have essentially the structure of 

 broad capillaries, from which they differ in that they arise from a single 

 vessel. They are therefore wholly venous or wholly arterial. 



Capillary circulation arises by the union of vascular outgrowths 

 from two vessels, the blood in which flows in more or less opposite directions, 

 in other words, from an artery and a vein. The vessels to the lungs are at 

 first a slender blind branch from a part of the aorta, and another blind 

 outgrowth from the left atrium [auricle] of the heart. These extend 

 through a column of mesenchyma to the epithelial ramifications of the 

 lung, over which they branch and become united. The blood flows to 

 the lung through the pulmonary artery, passes into capillaries and returns 

 to the heart through a vein. A similar circulation is shown in the diagram, 

 Fig. 150. It is essentially an arterio- venous circulation. From their 

 mode of development, capillaries have more connective tissue around them 

 than the sinusoids. 



A glomerulus is a round encapsulated knot of small subdivisions of 

 an artery which reunite before leaving the capsule, and soon after form 

 capillaries. Glomeruli occur in the kidney and mesonephros. They 

 are probably to be regarded as encapsulated capillaries rather than as 

 sinusoids. 



All the blood vessels of the young embryo, including the aorta and the 

 heart, are merely endothelial tubes. Capillaries and certain sinusoids 

 retain this structure in the adult, but the larger vessels have thick walls 

 formed by transformation of the surrounding mesenchyma. The wall of the 

 larger vessels consist of three coats or layers; the tunica intima, which is the 

 endothelium with a thin layer of elastic connective tissue; the tunica media, 

 which is chiefly smooth muscle with elastic substance intermingled; and 

 the tunica externa [adventitia] which is a dense layer of elastic connective 

 tissue sometimes containing muscle. In the heart the intima is called 

 endocardium; the media, myocardium; and the externa, which there is 

 covered with the pericardial mesothelium, is the epicardium. Capillaries, 

 arteries, veins, and the heart will be described in order. 



