SALIVARY GLANDS AND PANCREAS. 549 



both of mother and infant, which occasionally results from the moderate em- 

 ployment of them. The influence of various Medicines upon the Milk, is 

 another important question, which has not yet been sufficiently investigated. 

 As a general rule, it appears that the most soluble saline compounds pass into 

 the milk as into other secretions ; but there are many exceptions. Common 

 salt, the sesquicarbonate of soda, sulphate of soda, iodide of potassium, oxide 

 of zinc, tris-nitrate of bismuth, and sesquioxide of iron, have been readily de- 

 tected in the milk, when these substances were experimentally administered 

 to an ass ; and ordinary experience shows, that the human infant is affected 

 by many of these, when they are administered to the mother. The influence 

 of mercurial medicines taken by the mother, in removing from the infant a 

 syphilitic taint possessed by both, is also well known. The vegetable purga- 

 tives, especially castor oil, senna, and colocynth, have little effect upon the 

 milk ; hence they are to be preferred to the saline aperients, when it is not 

 desired to act upon the bowels of the child. 



V. Salivary Glands and Pancreas. 



693. The structure of the Salivary Glands and Pancreas in Man, bears 

 considerable resemblance to that of the Mammary glands. In some of the 

 lower tribes, however, they are much simpler. 



Thus, in the Echinodermata and in Insects, the Fig. 162. 



Salivary glands have the character of prolonged 

 coeca, more or less convoluted ; and the Pancreas 

 of Fishes presents itself in the form of a cluster of 

 short coeca round the pyloric extremity of the sto- 

 mach, and opening into it by distinct orifices. The 

 accompanying figure will give a sufficient idea of 

 the structure of these glands in Man ; the vesicles 

 are very minute, having a diameter only about 

 three times greater than that of the capillary blood- 

 vessels. Their development commences from a 

 simple canal, sending off bud-like processes, which 

 opens from the mouth, and lies amidst a cellular 

 blastema. As development proceeds, the canal be- 

 comes more and more ramified, increasing at the 



,, -I i i , . , . Lobule of parotid gland of a 



expense of the blastema, which is at last almost new . borninfant ejected with mer- 



wholly absorbed ; so that the substance of the gland cury . Magnified so diameters. 



consists of the ducts, with their ramifications and 



follicular terminations, and of the blo<5d-vessels which are distributed upon 



these. 



694. The salivary secretion is by no means necessarily constant, being 

 almost or completely suspended by cessation of the movement of the masticator 

 muscles and tongue, if unexcited by any nervous stimulus. Hence it is, that 

 the secretion is checked during sleep ; so that, if the mouth be kept open, its 

 surface is almost dried up by the atmosphere. The mode in which the secre- 

 tion is excited through the influence of the nervous system has already been 

 considered ( 425-6). The quantity of saliva formed during the twenty-four 

 hours has been estimated at about 15 or 20 ounces ; but on this point it is evi- 

 dently impossible to speak with certainty. The fluid obtained from the mouth 

 is of a more viscous character than the true saliva secreted by the glands, being 

 mingled with mucus. The salivary fluid varies as to its chemical re-action ; 

 being sometimes slightly acid, and sometimes slightly alkaline ; but it is seldom 

 precisely neutral. According to Huenefeld, it will at the same time strike a 

 blue colour with reddened litmus paper, and turn blue litmus paper red ; but 



