SECRETION. 261 



an arrangement is lacking. It has been shown than when the medulla is 

 separated from the cord by a section in the cervical or thoracic region the 

 action of dyspnoea, or of various sudorific drugs supposed to act on the cen- 

 tral nervous system, may still cause a secretion. On the evidence of results 

 of this character it is assumed that there are spinal sweat-centres, but whether 

 these are few in number or represent simply the various nuclei of origin of the 

 fibres to different regions is not definitely known. It is possible that in addi- 

 tion to these spinal centres there is a general regulating centre in the medulla. 



Mammary Glands. 



The mammary glands are undoubtedly epidermal structures comparable in 

 development to the sweat- or the sebaceous glands. Whether they are to be 

 homologized with the sweat- or with the sebaceous glands is not clearly deter- 

 mined. In most animals they are compound alveolar glands, and their acinous 

 structure and the rich albuminous and fatty constituents of their secretion 

 would seem to suggest a relationship to the sebaceous glands. But the histo- 

 logical structure of the alveolus with its single layer of epithelium points 

 rather to a connection with the sweat-glands. Whatever may have been their 

 exact origin in the primitive mammalia, there seems to be no question that 

 they were derived in the first place from some of the ordinary skin-glands 

 which at first simply opened, without a distinct mamma or nipple, on a defi- 

 nite area of the skin, as is seen now in the case of the monotremes. Later 

 in the phylogenetic history of the gland the separate ducts united to form 

 one or more larger ones, and these opened to the exterior upon the protrusion 

 of the skin known as the nipple. The number and position of the glands 

 vary much in the different mammalia. In man they are found in the thoracic 

 region and are normally two in number. The milk-ducts do not unite to 

 form a single canal, but form a group of fifteen to twenty separate systems, 

 each of which opens separately upon the surface of the nipple. Before preg- 

 nancy the secreting alveoli are incompletely formed, but during pregnancy 

 and at the time lactation begins the formation of the alveoli is greatly acceler- 

 ated by proliferation of the epithelial cells. 



Composition of the Secretion. — The general appearance and composi- 

 tion of the milk are well known. Microscopically milk consists of a liquid 

 portion, or plasma, in which float an innumerable multitude of fine fat-drop- 

 lets. The latter elements contain the milk-fat, which consists chiefly of neutral 

 fats, stearin, palmitin, and olein, but contains also a small amount of the fata 

 of butyric and caproic acid as well as slight traces of other fatty acid emu- 

 pounds and small amounts of lecithin, cholesterin, and a yellow pigment. Upon 

 standing, a portion of these elements rises to the surface to form the cream. The 

 milk-plasma holds in solution important proteid and carbohydrate compounds 

 as well as the necessary inorganic salts. The proteids are casein, belonging to 

 the group of nucleo-albumins ; lactalbumin, which closely resembles thesenun- 

 albumin of blood, and lacto-globulin, which is similar to the paraglobulin of 

 blood : the two latter proteids occur in much smaller quantities than the casein. 



