HUMAN PHYSIOLOGY. 



similar oasei, must be to consider the structure of the organs, 

 and tlion afterwards describe their functions or ues. The 

 parts oonoernod in digestion may be conveniently divided into 

 two the alimentary canal itself, and various organs supple- 

 mentary thereto. We will first examine the alimentary canal. 

 This i a long tube, measuring in the adult nearly thirty feet. It 

 commences at the mouth, and terminates at the lower extremity 

 of tho l>owol at the anus. It varies very much in sire in different 

 parts of its course, but has certain character* which are common 

 to it throughout its whole extent. Its walls are formed externally 

 of muscular fibre, and internally by a membrane which, from its 

 ii of secreting mucus, is called the mucous membrane. 

 This membrane i always present where the external and in- 

 ternal surfaces of the body communicate, and is a continuation 

 inwards of the skin, which it closely resembles in structure, 

 being but slightly modified to adapt it to its different office. The 

 analogy between the two is well shown by the fact, that where 

 skin, as in certain surgical operations, has been brought into the 

 position of mucous membrane, it has become gradually changed, 

 so as to perform the duties of the membrane it replaced. The 

 mucous membranes differ according to the positions in which 

 they are found, but all consist of a thin membrane spread over a 

 highly vascular tissue, and supporting a layer of cells which have 

 the power of secreting, or separating from the blood, the pro- 

 tective fluid mucus from which the membrane derives its name. 

 Mucus, as generally met with, is a peculiarly viscid fluid, trans- 

 parent and colourless, or nearly so, not soluble in water, and as 

 a rule sinking to the bottom ; alkaline in reaction, containing 

 about 6 per cent, of solid matter. Its purpose is, as has been 

 said, to protect tho surfaces by which it is secreted, and in health 

 the quantity is only just sufficient to answer that end ; but if the 

 membrane be inflamed, whether by cold or by the presence of 

 some foreign substance, the secretion is largely increased, and if 

 the irritation be continued, it eventually changes its character and 

 becomes purulent. To increase the secreting surface all mucous 

 membranes present pits or depressions some simple and shal- 

 low; others, deep tubes, straight and single, or convoluted and 

 compound. Those are the simplest form of what are called 

 secreting glands ; the larger glands do, in fact, only differ from 

 them in size and complexity of structure. 



The commencement of the alimentary canal the mouth is a 

 nearly oval-shaped cavity, enclosed by the muscular walls of the 

 cheeks, and the bony walls of the upper and lower jaw-bones. Its 

 roof is made by the upper jaw-bones in conjunction with the 

 palate-bones, whilst its floor is formed of the muscular tongue 

 and tho inner surface of the lower jaw, covered by mucous mem- 

 brane. It communicates behind with the pharynx, and contains 

 some parts of great importance in connection with tho function 

 we are considering the tongue, the teeth, the palate, and 

 several glandular structures. The tongue and palate are two of 

 the principal factors in the first part of the act of swallowing, 

 but as they have already been considered in our papers on 

 " Animal Physiology" (Vol. I., p. 321), we may pass on at once 

 to consider the teeth. 



In the human being, it is well known, the teeth appear in two 

 distinct sets the milk, or temporary, and the permanent. The 

 milk-teeth consist of twenty, ten in each jaw ; they begin to 

 make their appearance when the child is about six months old, 

 and are generally complete at the age of two years. The per- 

 manent teeth are thirty-six in number, and replace the temporary 

 set, commencing to appear about the ago of six, and not being 

 complete till tho eighteenth year, and very often not till a much 

 later period. The permanent teeth are distributed equally 

 between tho jaws, and consist of four incisors, or cutting teeth, 

 placed in tho centre ; two canine (dog-teeth), one on each side of 

 the incisors ; four bicuspids, next to the canine ; and six molars 

 situated farthest backwards the last molar tooth on each side, 

 in consequence of its late appearance, is known by the name of 

 " dens sapientiae," or wisdom-tooth. Every tooth consists 

 anatomically of three parts the crown, the part which projects 

 above tho jaw ; the fang, that portion which is inserted into 

 tho socket of the jaw ; and tho nock, the narrow portion joining 

 tho previous two. Into the structure of teeth three substances 

 enter, all modifications of bone tissue, differing mainly in the 

 relative proportion of animal and mineral elements, and con- 

 sequently in their degrees of density and hardness ; they are 

 dentine, or ivory, which holds the middle place as to hard- 

 ness, consisting of about 28 per cent, animal to 72 of earthy 



matter, and forms the greater part of the tooth, both crow* 

 and fan*. When a Motion of tooth is mad*, the dentine 

 ill found to enclose a cavity into whioh pntfeeta 



nerves. It is the exposure of this, and it. irritation by foreign 

 substances, when the hard portion of the tooth is decayed, that 

 gives rise to the sxcruciaamg pain of toothache. Covering the 

 whole of the crown of the tooth, as far aa Hi junction with Urn 

 fang, is a thin layer of extremely dense material the enamel s 

 this is the hardest of any organic sahstsBM, and r >!>! of 

 961 parts of mineral to but 3| of animal matter. Prom 

 the termination of the enamal, the fang of the tooth is covered 

 by a layer of the softest of the three nuiaiMhisiil iilnmium 

 the oementnm. This very closely resembles bone in 



and density, and is most abundant boot the apex of the fang, 

 where it has a tendency in old people to increase irnaeii1ere.il tj. 

 and occasionally gives rise to great pain and inconvenience. 



The form of the teeth, and the arrangement of their component 

 structures, vary according to the food on which the *<ffj is 

 designed to live. In the carnivora, or flesh-feeders, the teeth 

 are sharp-pointed, adapted for tearing the food. In the glisil 

 nivora, or grass-feeders, the teeth are broad, aad are made p 

 of vertical plates composed of enamel with TittntiiBi. whioh 

 alternate with similar plates of dentine. The result of tUe 

 provision is, that owing to the differing degrees of hardness of 

 the two sets of plates, they wear unevenly, keeping the ear* 

 face of the tooth constantly roughened, and so better fitted to 

 triturate and reduce to pulp the herbage and grain on which 

 these animals feed. One class of animals which gnaw or nibble, 

 such as the hare, the rabbit, the mouse, etc., have large front 

 teeth, which are kept constantly sharp by the enamel being re- 

 striated to the anterior surface. The posterior surface, being 

 formed of dentine, wears more easily, so that these teeth always 

 present a surface like the cutting edge of a chisel. 



Resemblances to the teeth of both the *iitrml and vegetable 

 feeders are found in those of man, and the natural inference is 

 that he was designed to be omnivorous to live on a mixed diet. 



In connection with the month are three pairs of snnrnHiie; 

 glands namely, the parotids, situated immediately below and 

 in front of the opening of the external ear, and communicating 

 with the mouth by ducts, which open just opposite the second 

 molar tooth of the upper jaw ; the snbmaxillary, situate below 

 the jaw; and the snblingual, under the tongue. These hare 

 ducts which open into the mouth beneath the tongue. The 

 secretion of these three sets of glands is the fluid called saliva, 

 which differs considerably from the ordinary mucus secreted by 

 the mucous membrane of the month. In addition to assisting 

 the latter in moistening the mouth and lubricating the food, it 

 plays an important part in changing some of the constituents 

 of the food. This, as well as its composition, will be more fully 

 described when we come to speak of the changes the food under* 

 goes in its passage through the alimentary canal. 



Communicating with the month, but partly divided from it 

 by the hanging curtain of the soft palate, which during the act 

 of deglutition is raised, so as to close the hinder passages; of 

 the nose, is a conical-shaped cavity, the pharynx. In the space 

 thus called seven openings are seen : the month, the posterior 

 orifices of the nose, the Enstachian tabes, which are duct* 

 leading to the internal ear ; the larynx, or windpipe, jiioieetod 

 by its little leaf -like cover, the epiglottis ; and behind the last 

 the oesophagus, or gullet. The oesophagus is about nine inches 

 in length, and extends from the pharynx to the stomach ; its 

 direction is towards the left side ; it passes downwards through 

 the thorax, and, perforating the diaphragm, enters the stomach 

 at its left upper extremity, just at which point the muscular 

 fibres of the oesophagus are arranged in a circular form, so as 

 to keep the opening closed save when food is being swallowed, 

 or during the act of vomiting. 



The stomach, which is the most widely dilated part of tb* 

 canal, varies much in size in different individuals. In shape it 

 is a conical bag twisted on itself, and, like the remainder of the 

 canal, has an external muscular wall and an internal mooons 

 memorano. It is extremely elastic, to enable it to adapt itself 

 to its various degrees of fulness, and is situated in the abdomen 

 rather towards the left side, and in immediate contact with its 

 anterior wall. It is also in relation with the left lobe of the 

 liver, the pancreas, the spleen, and the heart. From the 

 latter it is separated by the diaphragm. Its approximation to 



