HISTOLOGICAL DEMONSTRATIONS. ioi 



non-striped muscular fibres mostly circular in their arrange- 

 ments, with elastic and white fibrous tissue superadded in 

 vessels of considerable size. In very large vessels, such as 

 the aorta, there is a great preponderance of elastic elements 

 in the form of elastic membranes and fibres. The outer 

 coat is made up of elastic and white fibrous tissue. In 

 arteries and veins the same structural elements are gene- 

 rally present, the veins having only thinner walls than the 

 arteries. 



a. Search for vessels that appear about an inch in 

 breadth when seen with the power we are at present 

 employing (300 diam.) In the outer coat the wavy fibrous 

 tissue will be readily recognised. Inside the outer tunic an 

 appearance of rounded cells will be seen, provided the 

 vessel under examination be an artery; if not, shift the 

 preparation and search for a vessel with this characteristic. 

 This appearance of rounded cells is due to the muscular 

 fibres seen through their long axes as they curl round the 

 vessel. The nucleus appears as a bright refracting spot, if 

 the part of the fibre seen through happen to contain it. 

 With careful focussing the margins of these rounded bodies 

 at the periphery are seen stretching across the vessel as the 

 outlines of the muscular fibres. In the smallest vessels 

 there is only a single layer of muscular fibres, but in those 

 of larger size two and even three layers may be found. 

 The elastic membrane will be more readily seen when the 

 outer and middle coats have been rendered transparent by 

 acetic acid. 



b. Add acetic acid. The tunica adventitia swells up, 

 becomes transparent, and the nuclei of its connective tissue 

 corpuscles appear. The muscular fibres become very 

 transparent, and show their more or less staff-shaped nuclei 

 crossing the vessel. With deeper focussing other nuclei 

 may be seen, usually of an oval shape, with their long axes 

 generally parallel with that of the vessel. These belong to 

 the epithelial lining. 



The elastic membrane is composed of a transparent 

 highly refractile elastic substance. It may be of uniform 

 extension or fenestrated. It is generally marked by ir- 



