THE RABBIT. 87 



The mucous layer consists of (1) the lining epithelium, (2) a layer 

 of adenoid or lymplioid tissue, containing abundance of blood- 

 vessels, and (3) a double muscular layer (niwcularis mucosce) con- 

 consisting of an inner layer in which unstriped fibres are arranged 

 so as to tend to encircle the intestine (circular muscles) and an outer 

 in which they are arranged longitudinally. The whole of the folds 

 into which the inner surface of the intestine is thrown, affect the 

 mucous layer only. Where there are glands, like the peptic glands 

 of the stomach, the epithelium is sunk down into the adenoid 

 tissue ; where there are villi, the adenoid tissue runs up into them. 



The sub-mucous layer is a layer of connective tissue containing the 

 main blood-vessels, lymphatics and nerve-fibres from which the 

 other layers are supplied. It is not affected by any of the folds into 

 which the mucous layer may be thrown. 



The muscular layer is just like the muscularis mucosae, but much 

 thicker there is an inner circular layer, and an outer longitudinal 

 all of unstriped fibres. It is these layers that cause the peristaltic 

 movement of the intestine, and the analogous movements of the 

 stomach-wall during gastric digestion. There is a layer of nerve- 

 fibres between the two sets of muscle-fibres. 



4. Structure of Blood-vessels. Capillaries have walls consist- 

 ing of endothelium only, and this same endothelium forms a con- 

 tinuous lining to the whole of the vascular system veins and 

 arteries, chambers of the heart, and lymphatics. In the veins and 

 arteries there are added to this endothelium three coats (1) a layer 

 of connective tissue with great abundance of yellow elastic fibres 

 the elastic layer, (2) a layer of unstriped muscle-fibres, arranged 

 mostly in a circular manner, (3) an outer layer of connective tissue, 

 also with some elastic fibres. In veins the muscular layer is much 

 thinner than in arteries ; the reason for this lies in the function of 

 the muscular layer, which is to contract or relax as may be needed, 

 and so decrease or increase the calibre of the artery and therefore 

 the amount of blood going to the part of the body served by that 

 artery. In veins, where the flow of blood is from small vessels to 

 large, there is obviously less need for the muscular layer. 



5. Muscles of Heart. At the points where the walls of the 

 large vessels merge into that of the heart, the endothelium continues 

 unchanged, but the other layers undergo great change. The inner 

 and outer connective-tissue coats lose their elastic character, and the 

 muscular layer becomes enormously thickened, especially in the 

 ventricles. At the same time, a change in the form of the muscle- 

 fibres takes place ; they are no longer ordinary unstriped fibres, but 

 partake of a character intermediate between striped and unstriped. 

 Each fibre is still a single cell, with a single nucleus, but is rather 

 oblong than spindle-shaped, and gives off branches which join it to 

 adjacent fibres. Moreover, it has a distinct transverse striation, 

 though not quite as definite as that of true striped muscle (fig. 49, B). 



