The essential parts of every secretory apparatus are a simple membrane, apparently textureless, named the _primary_, or _basement membrane_, certain cells and blood-vessels. The serous and mucous membrane are examples. B. The division and description of the different membranes and tissues are not well defined and settled by anatomical writers. This is not a material defect, as a clear description of the different parts of the system can be given by adopting the arrangement of almost any writer. C. FAT is one of the non-nitrogenous substances. It forms the essential part of the adipose tissue. Chemical analysis shows that all fatty substances are compounds of carbon, hydrogen, and oxygen. They are lighter than water, generally fluid at the natural temperature of the body, and burn with a bright flame, forming water and carbonic acid. CASEINE is abundantly found in milk. When dried, it constitutes cheese. Alcohol, acids, and the stomach of any of the mammalia coagulate it; and it is also soluble in water. It is found in the blood, bile, saliva, and the lens of the eye. CHONDRINE is a variety of gelatin. It is obtained from cartilage. It is soluble in warm water, but solidifies on cooling. LACTIC ACID is common to all the solids and fluids of the system. It is found united with potash, soda, lime, or magnesia. D. The word _duodenum_ is derived from the Latin, signifying "twelve," since the intestine, of which this is the name, is usually about twelve fingers' breadth in length. The _jejunum_ is also from the Latin _jejunum_, empty, since it is usually found in that condition after death, as the food seems to pass rapidly through this part of the intestine. The term _ileum_ is from the Greek, signifying "to twist," since it always appears in a contorted condition. The name _cæcum_ is derived from the fact of its being a blind or short sack, perforated by the extremity of the ileum. The name of the next division of the intestine--_colon_--is from the Greek, "to prohibit," as the contents of the alimentary canal pass slowly through this portion. The _rectum_ is named from the straight direction that it assumes in the latter part of its course. E. The food is forced through the alimentary canal by contractions of its muscular coat, produced by the nervous filaments of the sympathetic system, not being at all dependent on the cerebro-spinal centre. This is called the peristaltic, or vermicular motion. The great length of intestine in all animals, and especially in the herbivorous ones, is owing to the necessity of exposing the food to a large number of the lacteals, that the nourishment may all be taken from it. F.