LESSON 52.] 



NUTKITION IN MAMMALIA. 



187 



FIG. 301. 



artery, with its three distinguishing characteristic coats, ends ab- 

 ruptly, suddenly, in a capillary ; the structure is altered, and so is 

 the character of the circulation. In like manner, a capillary sud- 

 denly terminates, and a vein, with its characteristics of structure 

 and function, commences, and so with other tissues : each maintains 

 its own integrity without compromise, and ends only at the point of 

 junction with another tissue. 



806. The duodenum of a dog is a most surprising sight. The 

 great length and breadth of the villi, conjoined with the large dis- 

 tribution of capillaries to each villus, renders the preparation under 

 the microscope a very gorgeous one. 



This is represented, as far as art can go, in Fig. 301, but the 

 white lines on a black ground form but a sorry 

 substitute for the rich color of the vermilion 

 which has been injected into the vessels to 

 simulate arterial blood. 



807. In the Jejunum (empty, and so call- 

 ed because this intestine is always found 

 empty), the form and size of the villi undergo 

 modification in all animals, and this is repeated 

 in the Ileum ; examples, however, are not ne- 

 cessary in this connection. 



808. In the colon, as shown before, the 

 structure of the stomach is so nearly repro- 

 duced that it is extremely difficult to define one tissue from another ; 

 in fact, the only rule appears to be the following : generally the gas- 

 tric cells of the stomach are wider on the surface, and deeper than 

 the cells of the colon, consequently it is easy to penetrate many of 

 the former by the microscope, and see the capillaries on the floor of 

 the cell, around the mucus-tubes. (See 



Fig. 299.) As there is no such struc- 

 ture in the cells of the colon, the attention 

 of the observer is necessarily restricted to 

 the surface, and here the identity with 

 the stomach appears to be perfect. 



The colon of the Dog is shown in Fig. 

 302, and fully justifies all that has been 



Said f i*' Colon of the Dog. 



Enough has now been written, and 



examples sufficient have been produced, to show the principles of nu- 

 trition, as displayed in the animal kingdom ; those subjects which 



Villi of the duodenum, 

 Dog. 



FIG. 302. 



