49 



BLOOD-VESSELS. 



There are three well-marked varieties of blood-vessels arteries, veins, and capillaries. A 

 typical artery has three coats outer, middle, and inner. In a middle-sized artery the inner coat 

 consists of a layer of elongated squames with their long axis corresponding to the long axis of 

 the vessel ; outside this, delicate connective tissue and an elastic membrane or lamina, either 

 with holes in it ' fenestrated membrane ' or it consists of a sheet of elastic tissue which in 

 sections is thrown into folds, owing to the contraction of the muscular coat outside it. It is 

 to be remembered, however, that these folds do not exist in a living vessel distended with 

 blood. The middle layer consists of circularly-disposed non-striped muscular fibres, with a 

 small amount of elastic and white fibres. The outer coat (adventitia) consists of fibrous 

 tissue with longitudinally disposed elastic fibres, which are specially numerous next the 

 muscular layer. In the largest arteries (aorta) there is relatively a large amount of elastic 

 tissue in the inner and middle coats, and in the middle coat the non-striped muscle 

 and elastic plates occur in alternate layers. In the veins the division into coats is not 

 so sharply marked. The middle coat contains less muscular fibre-cells and more connective 

 tissue, and the outer coat not unfrequently contains non-striped muscle, especially in large 

 veins. The capillaries consist of a layer of nucleated, flattened, epithelial cells, joined at their 

 edges with a cement-substance. 



Small arteries and capillaries are best studied in the pia mater. 



SMALL ARTERIES AND CAPILLARIES. 



PREPABATION. Get the head of a recently killed sheep, and remove the brain. With a 

 scalpel remove as much of the brain as possible, leaving the pia mater. With a stream of 

 salt solution wash away the remainder of the brain-substance. It is most important to 

 remove all the brain-substance, else it forms a granular deposit which obscures the prepara- 

 tion. Place the pia mater in chromic acid and spirit solution for a week. After washing it 

 thoroughly in water, snip out a small piece and stain it with logwood, and mount it in dammar. 

 A similar preparation may be stained with picrocarmine, and mounted in Farrant's solution. 



EXAMINATION (L). Select a small artery, and study its mode of branching and how 

 these branches terminate in capillaries. Note in the artery the transversely disposed nuclei of 

 the muscular fibres of its middle coat (PL IX., Figs. 6 and 7). Observe the outer coat. Search 

 for a vein with its much thinner coat and fewer transversely disposed nuclei i.e. muscular 

 fibres and compare it with an artery. 



(H). Select a small artery. Observe its outer and middle coats, and, composing the latter, 

 non-striped muscular fibres, disposed in one or more layers. The fusiform nuclei of the 

 muscular coat are very apparent. The folds of the elastic lamina of the inner coat may be 

 seen. Study a capillary. It appears as a perfectly structureless membrane, with oval nuclei 

 imbedded in it, which bulge into its lumen. It requires the use of nitrate of silver to demon- 

 strate its endothelial characters. 



H 



