546 THE BOOK OF THE DOG. 



these little pits, all opening into a common reservoir, and having attached to this a common 

 duct to carry its secretion where it is needed, would give us a higher form of gland, where we 

 would have a large extent of secreting surface at the expense of very little room. Now, let us 

 go a step higher, and we find a gland, like the parotid for instance, which is formed thus : 

 Instead of little pouches, all opening into a common reservoir, as in a more simple gland, 

 imagine innumerable little bags, each with a tiny pipe or stalk, and all those pipes or stalks 

 opening into each other to form rather larger pipes, and going on joining others, until they 

 finally form one large tube, which conveys the secretion to its destination, as the duct of the 

 parotid conveys the saliva to the mouth. 



But where, it may be asked, does the secretion come from ? Why, from the blood. And 

 every little grape-like bag or pouch, the whole of which together unite to form that great 

 conglomerate gland, the parotid, is surrounded by capillary arteries and veins. Now, all 

 the little grapes, as it were, are united together by connective tissue, and imbedded, in the 

 case of the parotid, in fat. If we can understand the construction of the parotid gland, we 

 have tlie key to every secreting gland in t/te body salivary glands, kidneys or urinary glands, 

 testicles or semenary glands, and the great liver itself, the biliary gland, which, with its net- 

 work of blood-vessels, &c., is the most complicated gland in the body. The appearance of a 

 bunch of grapes gives one a good, though rough, example of the shape and mechanism of a 

 large gland. The grapes themselves represent the single secreting glands or pouches, the small 

 stalks their ducts, and the large or main stalk the final or efferent duct. 



The alimentary canal may be compared throughout its whole length to a tube, wider at 

 some parts than at others ; it has three coats, that is it is in reality three tubes one inside the 

 other. Having already shortly mentioned the lining coat the mucous membrane it will 

 suffice for our purpose at present to mention the muscular coat. The fibres, then, of this 

 muscular coat are for the most part circular, that is they are in concentric rings, but in some 

 portions of the canal they are transverse as well, as in the stomach ; the bearing of which will 

 presently be seen. 



Muscles and Muscular Contraction. Muscle, in plain language, is flesh. Each muscle is 

 composed of a bundle of fibres held together by connective tissue, which, if examined by tearing 

 the fibres of a bit of raw meat asunder, we find not unlike cobwebs. It is this tissue, this 

 cobwebby tissue, that binds all the different portions of the soft organs of the body together. 

 Muscles are meant for motion. In addition to the blood-vessels which supply them with 

 nutriment, they are supplied with nerves to enable them to contract. To contract means to 

 draw together, to shorten. A muscle always contracts in its long axis. If one wills to 

 contract his biceps, for exampje, he shortens it, and consequently thickens it ; and one tendon 

 of the biceps muscle being situated in the forearm and the other in the shoulder, by contracting 

 the biceps the forearm is either drawn up, or, if the hand is fixed in say an iron ring in the 

 ceiling contraction or shortening of the biceps will draw the body upwards. 



There is not a function of the body in which muscular contraction does not play an 

 important part. 



Now, as muscles always contract in relation to their long axis, if a tube is encircled by a 

 muscle, the contraction of that muscle would evidently result in a narrowing of the diameter of the 

 tube, and the contents of the tube would be forced onwards. 



The muscles of the alimentary canal are presided over by nerves, over which the animal has 

 little or no control. The mere contact of the food upon the lining membrane of a healthy digestive 

 canal, however, is sufficient to cause alternate contractions and dilatations of the muscular coat 



