THE VESSELS. 287 



much the more minute, the larger the consumption of blood (the narrowest 

 network is in the lungs, the glands of the skin, the mucous membranes, &c.) 

 According to the shape of the space bounded by them, we may distinguish 

 roundish or oval (e. g. in the lungs, the corium, the glands), and longitudinal 

 or elongated meshes (in the nerves and muscles). Ball-shaped, convoluted 

 meshes are only found in the cortical substance of the kidneys (e. g. glomeruli 

 Malpighi). 



Function. The capillary vessels impart, through their thin (not porous) 

 walls, the nutritious material of their blood to the organs, in which they 

 spread out and ramify, and on the other side they take up, likewise (in the 

 lungs), the inspired oxygen. Vasa exhalantia, exhaling vessels. 



524. II. Pulsating vessels, arterice, 



are the vessels, the trunks of which arise from the ventricles of the heart, and 

 which distribute the blood throughout the body. Their walls are thicker, 

 and more elastic than the rest of the vessels. The smaller arteries possess, 

 relatively, the strongest parietes ; the thinnest arteries are those of the cranial 

 cavity. They pulsate, that is, they extend in length and breadth, when the 

 ventricles of the heart contract (systole), and thereby force their blood into the 

 arteries, and they again become narrower and shorter, when the ventricles of 

 the heart dilate (diastole). The arteries are further distinguished by their 

 great elasticity and contractility. The first produces retraction when the ar- 

 teries are divided across ; the last a narrowing, but not collapse, as happens 

 in the veins. Of the coats, before all, we remark : 



That consisting of circular fibres (the so-called middle, elastic coat). It is 

 thick, yellowish white, dry, and fragile, and the medium of contraction. It 

 is distinguished from the muscular tissue by its easy solubility in nitric, inso- 

 lubility in acetic acid, and by yielding gelatine on boiling ; from the fibro-cel- 

 lular or uniting tissue, by insolubility in boiling acetic acid, difficult solution 

 in mineral acids, caustic potash, and gastric juice. 



The external, properly elastic coat of arteries, is firm, and stronger than in 

 the veins, and is remarkable for not dividing when an artery is included in a 

 ligature, which the internal coat does. 



The inner coat, tun. vasorum comm., that is, the striated tunic (for the tunic 

 of longitudinal fibres is usually wanting), is only in a diseased condition as 

 thick as we are accustomed to find it, especially in old people, when it is so 

 much the more fragile. 



The arteries take a serpentine course, generally deeper than the veins and 

 lymphatics, the larger on the flexion side of the joints, and convey (the pul- 

 monary excepted) bright red blood. 



525. III. Blood-vessels, vena^ veins, 



originate from the capillaries, and open into the auricles of the heart. Their 

 walls are thinner, and more lax, and when the vessel is empty, they falltoge- 

 ;her, as the elastic coat is entirely wanting in them ; the contractile, circular, 



