312 THE ADRENAL, GENERAL MOTOR, AND VAGAL SYSTEMS. 



plexus of Meissner, which closely resembles Auerbach's nervous 

 net-work within the muscularis, possessing, however, smaller 

 ganglia and somewhat closed meshes. From the plexus of the 

 submucous tunic fibers pass into the mucosa, to form net-works 

 about the glands and to send fibrillse into the villi." 



In analyzing the functions of the intestinal tract it is im- 

 portant to note that two distinct sets of active structures are 

 present: (1) the secreting glands of Lieberkiihn and of Brun- 

 ner; (2) the villi of the agminated lymph-follicles (Peyer's 

 patches) and the solitary lymph-follicles. 



Secreting Glands. The glands, or crypts, of Lieberkiihn, 

 found in close array throughout the entire length of the intes- 

 tine, including the colon, are present only in the upper, or 

 mucous, layer. They are simple in construction and recall the 

 sweat-glands, minus the coils and muscle: i.e., a net-work of 

 capillaries and probably nerve-fibrils overlying a delicate base- 

 ment-membrane which in turn surrounds a single layer of 

 columnar epithelial cells. These cells radiate toward a com- 

 mon center and thus form a minute tube which opens upon the 

 mucous membrane between the villi. Their functional mech- 

 anism is doubtless that of all simple tubular glands. As sym- 

 pathetic fibers are alone found here, we are bound to accept 

 them as motor fibers. Yet only two sets of fibers are necessary: 

 a set to the glandular arterioles deprived of capillaries; an- 

 other to the secretory cells. During activity the first set (the 

 "extrinsic vasoconstrictors") reduces the caliber of the arte- 

 rioles, forcing more blood into the capillaries, while the second 

 set incites and governs the activity of the secreting cells. 



Howell refers to the crypts of Lieberkiihn as follows: 

 "These structures resemble the gastric glands in general ap- 

 pearance, but not in the character of the epithelium.. The 

 epithelium lining the crypts is of two varieties: the goblet 

 cells, whose function is to form mucus, and columnar cells with 

 a characteristic striated border. . . . Whether or not the 

 crypts form a definite secretion has been much debated. Phys- 

 iologists are accustomed to speak of an intestinal juice, 'succus 

 entericus/ as being formed by the glands of Lieberkiihn; but 

 practically nothing is known as to the mechanism of the secre- 

 tion." We have seen that nerve-filaments are distributed to 



