This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2293). In a few species, in addition to the salivary glands met with in Man *, there is a group, apparently a continuation of the molar, which mounts up along the superior maxillary bone, beneath the zygoma, even to behind the globe of the eye. The excretory ducts derived from this group pierce the mucous membrane near the posterior margin of the superior alveolar ridge. Such an arrangement is met with in the Ox, the Sheep, and the Horse.
(2294). In the Amphibious Mammalia the salivary system is very feebly developed; and in the Cetacea, as might be expected from their habits, no salivary glands whatever are to be detected.
(2295). Before considering the mechanism of deglutition in the Mammalia, we must, in the next place, briefly describe their hyoid apparatus; more especially as this remarkable system of bones, which in the lower Vertebrata was so importantly connected with the respiratory function, is now reduced to an extremely simple condition, and, although it is still intimately connected with the larynx, is more particularly remarkable as forming a centre of attachment for almost all the muscles of the throat.
(2296). Perhaps there is no part of the bony framework of the body that exemplifies more strongly than the os hyoides the impossibility of attaining correct physiological views relative to the composition of the skeleton by the mere examination of the human subject. Let the student, for instance, compare for a moment the os hyoides of Man with that of the Fish, or of the Amphibious Reptile, and endeavour, in the simple segment of a circle presented by the one, to find the analogues of the body and complicated arches of the others; then, doubtless, he will find that, without some intermediate gradations of form, it is not easy to trace the slightest relationship between them.
(2297). The human os hyoides consists of a central portion and two cornua; but these are generally so completely consolidated as to form but one bone, which is connected by the interposition of a broad ligament with the upper margin of the thyroid cartilage; moreover two smaller appendages, called the lesser cornua, are articulated with the upper surface of the hyoid bone, close to the point of junction between the cornua majora and the body, from whence ligaments, called the stylo-hyoid, pass upwards and backwards to the styloid processes of the temporal bone.
* Lecons d'Anat. Comp. iii. p. 210.
(2298). All the apparatus of hyoid arches passing between the body of the bone and the base of the cranium, which were so largely developed in the lower Vertebrata, have therefore totally disappeared; and the question to be solved is, how we may identify the remaining portions with any of the elements of the more complex structures that have come under our notice.
(2299). Difficult as this would be to the student who had confined his attention to the human body, on referring to the os hyoides of a quadruped, one of the Carnivora for instance, the analogies become at once perceptible. The body (fig. 401) is evidently the representative of the central portion of the hyoid apparatus in Fishes, in Reptiles, and in Birds, which have been described in preceding pages. The lingual elements, found even in Birds, are quite obliterated; but two arches still remain. The posterior of these(fig.401), which represent the larger cornua of the human os hyoides, do not reach the cranium, but, as in Man, are attached by muscle and ligament to the thyroid cartilage; while the anterior cornua, so small in Man, are in quadrupeds by far the largest, each consisting of two pieces, of which the second are articulated with the extremities of the styloid bones, and these last are in turn joined to the temporal bones by means of articulating surfaces. In Man the styloid bones become anchylosed with the temporal, giving rise to the "styloid processes;" and the intermediate pieces of the anterior cornua have their places supplied by ligaments (the stylo-hyoid): in this way, therefore, the hyoid apparatus attains the form that it exhibits in the human skeleton.
(2300). The muscles connected with the os hyoides in quadrupeds correspond with those met with in the human body; and their action in effecting the deglutition of food is well known to the anatomical reader.
(2301). The passage of the fauces in the Mammalia presents an organization peculiar to the class, and exhibits structures adapted to prevent alimentary materials from entering the air-passages during the operation of swallowing. The most remarkable of these is the epiglottis, forming a valvular fibro-cartilaginous lid, that accurately closes the opening of the larynx during the transit of food into the throat. The communication between the posterior nares and the faucial cavity is likewise protected by a musculo-membranous valve, called the velum pendulum palati; but as, with the exception of the Cetacea, hereafter to be noticed, the arrangement of these parts exactly resembles what is seen in the human subject, it would be superfluous to describe them more minutely in this place.
Fig. 401. Os hyoides.
(2302). The bag of the pharynx in all the Mammalia is similar in its structure to that of Man; and its muscles, namely, the stylo-pharyn-geus and the three constrictors, although stronger than in our own species, offer no differences worthy of more particular notice.
(2303). The oesophagus, leading from the termination of the pharynx into the stomach, is a long muscular tube that traverses the chest in front of the bodies of the dorsal vertebras, and, having pierced the diaphragm, reaches the abdominal cavity. Its lining membrane is loose and much plicated, so as to allow of considerable dilatation; but externally its walls are very muscular, the surrounding muscles being arranged in two distinct layers. In Man the outer stratum of muscular fibres is disposed longitudinally, while the inner layer consists of circular fibres; but in most other Mammalia both these layers assume a spiral course, and cross each other obliquely as they embrace the oesophageal tube.