THE INTESTINES. 91 



apertures of the glands ; it is more complicated or more 

 simple, according to the breadth of the interspaces of the 

 glands and the occurrence of elevations upon them, but seems 

 never to consist of simple vascular rings. From this network 

 the veins, which are relatively wide, arise by many radicles ; 

 they then, further apart from one another than the arteries, 

 and receiving no more blood, penetrate the glandular layer, 

 and upon the outer surface of the mucous membrane, enter, 

 often at right angles, a wide venous network with partly 

 horizontal vessels, in the submucous tissue. 



The lymphatics of the stomach form a superficial fine net- 

 work and a deep, coarse, one; and can only be demonstrated by 

 injection. The numerous small branches which pass from the 

 mucous membrane, their aggregation into larger trunks, and 

 final penetration of the muscular tunic, are readily seen in large 

 Mammalia, killed during digestion. The nerves of the stomach, 

 derived from the vagus and sympathetic, are readily traced into 

 the submucous tissue and they may also be observed entering 

 the muscular layer of the mucous membrane ; but it becomes 

 impossible to follow them further, principally because, in the 

 interior of the mucous membrane itself, they present no more 

 dark edged tubules, but probably only the pale embryonic 

 fibres. 1 



MUCOUS MEMBRANE OP THE SMALL INTESTINE. 



§ 152. 



The mucous membrane of the small intestine is thinner than 

 that of the stomach, but more complex in its structure, inas- 

 much as, besides the tubular or Lieberk'uhnian glands, it presents 



'[In his * Mikroskopische Anatomie,' B. II, 2, pp. 149,153,164, Professor 

 Kolliker shows that the muscular layer of the mucous membrane of the stomach 

 and intestine was discovered by Middeldorpf (De Glandulis Brunnianis, Vratisl, 

 1846, c. tab.) but remained unnoticed until it was re-discovered by Brucke and him- 

 self. In the small intestine there are, when this muscular stratum is well developed, 

 two layers, though they are not always complete; the external layer is composed of 

 longitudinal, the internal of transverse fibres. 



In the villi, the smooth muscular fibres have been found not only in many mammalia, 

 but also in birds. The contraction of the villi which they effect appears to have been 

 noticed by Lacauchie, Gruby and Delafond, so long ago as 1842. — Eds.] 



