THE BLOOD VESSELS 
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3. Pulmonary System. 
(a) Arteries (injected blue). 
Pulmonary artery, leading from the right ventricle, its 
right and left branches distributed to the correspond¬ 
ing lungs. 
(b) Veins (injected red). 
Pulmonary veins from the various lobes of the lungs into 
the left auricle. 
B. THE CAPILLARIES. 
For the study of capillaries, it is necessary to use either thick 
microscopic sections or small fragments of tissues in which the 
capillaries have been injected. Study with care and draw the 
details of a small region of one or more preparations , noting in each 
case the adaptation of the form of capillary network to the structure of 
the tissue in which it is located. 
The following preparations are suggested: 
1. Transverse section of intestine: Note the capillaries of the 
villi and of the muscular coats. 
2. Section of tongue: Distinguish the capillary network of the 
various sets of muscles. 
3. Section of lung tissue: Note that the capillaries of the 
alveoli form an almost continuous layer in the alveolar wall. 
4. Surface mount of small pieces of injected mucous membrane 
of rodent colon, showing capillary network in relation to short 
tubular glands. 
C. STRUCTURE OF WALLS OF BLOOD VESSELS. 
Study cross sections of veins and arteries and compare as to 
thickness of the walls and form of lumen. Note that in both there 
are three doats, the tunica intima, comprising the endothelial 
lining (supported, in the case of the artery, upon an elastic mem¬ 
brane), the tunica media, consisting of layers of elastic connective 
tissue and plain muscle fibers (this layer in the veins possesses less 
of the elastic and muscle tissue and is mainly of fibrous connective 
tissue), and the tunica adventitia, consisting mainly of elastic 
