68o HUMAN ANATOMY. 



from the blood, as the lungs or liver, as well as those producing substances directly 

 entering the circulation (organs of internal secretion), as the thyroid gland, are pro- 

 vided with exceptionally rich and close net-works. The mesh-works within the walls 

 of the pulmonary alveoli are of remarkable closeness and are often narrower than the 

 capillaries surrounding them. 



Under the name, sinusoids, Minot' has grouped the circulation occurring in 

 certain organs, as the liver, in which the capillaries are formed by the invasion and 

 subdivision of the large original blood-channel by the tissue-cords. The resulting 

 sinusoids differ from ordinary capillaries, therefore, in connecting afferent and efferent 

 vessels of the same nature, both being either venous or arterial. Capillaries, on the 

 contrary, form communications between arteries and veins. In consequence of the 

 invagination of the original vessel, its endothelium bears an unusually intimate rela- 

 tion to the tissue-trabeculae, little or no connective tissue intervening. F. T. Lewis ^ 

 has shown that the Wolffian body and the developing heart also present examples of 

 sinusoidal formation, and suggests the significance of sinusoids as representing a 

 primitive type of circulation. 



THE BLOOD. 



The fluid circulating within all parts of the blood-vascular system consists of a 

 clear, almost colorless plasma or liquor sanguhiis in which are suspended vast 

 numbers of small free corpuscular elements, the blood-cells. The latter are of two 

 chief kinds, the colored cells, or erythrocytes^ and the colorless or leucocytes. The 

 characteristic appearance of the blood is due to the presence of hemoglobin con- 

 tained within the erythrocytes which, while individually only faintly tinted, collect- 

 ively impart the familiar hue as well as a certain degree of opacity. That the 

 characteristic pigment is limited to the cells is shown by the lack of color and the 

 transparency of the plasma when examined under the microscope, although to the 

 unaided eye the blood appears uniformly red and somewhat opaque. The most im- 

 portant property of hemoglobin is its great affinity for oxygen which, taken up from 

 the air during respiration and combined as oxyhemoglobin, is carried by the red 

 cells to all parts of the body. When rich in oxygen (containing about twenty vol- 

 umes) the blood possesses the bright scarlet hue characteristic of arterial blood; after 

 losing approximately one-half of its oxygen and acquiring about an equal volume of 

 carbon dioxide during its intimate relations with the tissues, the blood returned by 

 the veins is dark purplish-blue in color. If the hemoglobin escapes from the eryth- 

 rocytes into the plasma, the latter becomes deeply tinged and the blood loses its 

 opacity and becomes transparent or ' ' laked. ' ' This discharge is known as he}?iolysis. 



The specific gravity of normal blood is about 1055 ; its reaction is alkaline and 

 due chiefly to the presence of sodium carbonate. Immediately after withdrawal from 

 the body the blood possesses a characteristic odor that probably depends upon cer- 

 tain volatile fatty acids. When fresh it is slippery to the feel, but after exposure 

 to air becomes sticky. Upon standing it undergoes coagulation, whereby the cor- 

 puscles become entangled among the innumerable delicate filaments of fibrin, a pro- 

 teid substance that appears in the plasma after withdrawal of the blood from the 

 body. As the result of this entanglement the corpuscles are collected into a dark- 

 colored, jelly-like mass, the blood-clot or crassamentum, that separates from the sur- 

 rounding clear straw-colored scriun. The latter possesses an alkaline reaction and 

 a specific gravity of 1028. The serum closely resembles the liquor sanguinis, con- 

 taining about ten per cent, of solid substances, of which about three-fourths are pro- 

 teids — serum-albumin, serum-globulin, and fibrin-ferment, the latter replacing the 

 fibrinogen present in the plasma before coagulation occurs. 



Blood-Crystals. — The chief constituent of the red cells, the hemoglobin, prob- 

 ably exists within the corpuscles as an amorphous mass in combination with other 

 substances (Hoppe-Seyler) from which it must be freed by solution before crystal- 

 lization can occur. After laking, the coloring matter of the blood, in the form of 

 oxyhemoglobin, separates into microscopic crystals that belong to the rhombic sys- 

 tem, usually appearing as elongated rhombic or rectangular plates (Fig. 643). 



' Proceedings Boston Soc. Nat. History, vol. xxix, 1900. 

 * Anatomischer Anzeiger, Bd. xxv., 1904. 



