Abstract of Lectures on the Structure and Functions of the Vertebrate Skeleton

W. K. Parker
1880 BMJ (Clinical Research Edition)  
of Anatomy in the College. LECTURE VI. THE aimiphibia are a subdivision of the icthyopsida, which, like the dipnoi, clevelop lungs as well as gills, but which often shed the latter, breathing only by the former. Their embryos, like those of the fish, develop neither amnion nor allantois. They are, indeed, what may be termed quasi-fish. There are four orders of this subclass, namely, the coecilians, the urodeles, the anura, and the labyrinthodonts, the last of these being the large extinct
more » ... large extinct amphibia of the coal-measures. The living forms of aml-phibia begin life as a sort of fishes, having gills; and, as a rule, live in the water till they acquire lungs, when they shed their gills; some, however, keep their gills, and continue to live in the water. In the urodeles and anura. or the tail-bearing and tailless amphibia, we find many things in common; they agree in possessing gills in their larval state, though different in character. The urodeles have, some for a while, and others throughout life, three pairs of pinnate external gills attached to the first three branchial arches, a single gill to each arch, there being generally a fourth arch which does not carry a gill. These are true inner branchial arches. In the frogs and toads, there are three important modifications of the branchial system. In the common kinds, there are three fur-tufted gills growing from the first two, at least, of ,the four branchial arches, all of which are functional. These are soon hidden by an opercular outgrowth from the hyoid arch, which covers over and closes up all the branchial region, leaving, however, a small aperture on the left side. The primary pharyngeal wall iiot only splits, so as to form four clefts on each side, but the wall itself becomes divided, so as to form a series of pouches, each of which has a cartilage within and a cartilage without the opercular skin, loosely covering the pouches outside. The intrabranchial arches are small bars, while the second and fourth are large pouches. Tufted vascular growths, like those first seen outside, growv on thc inside of the large pouches and bars, and also from the three branchial plefts outside the extrabranchials. These correspond to the external gil Is seen in unhatched skates and sharks. Suctorial cartilages have nearly disappeared in the embryos of urodeles, but in the batrachia they are nearly as much developed as in the lanmprey. In the urodeles, we find no trace of a gill on the first and seconid arches behind the mouth, nor on the sixtlh, which exists in the majority of the species. It is evideimt that the tailed ampliibia have been dropping from time to time parts no longer useful to them, whilst straining after a higher organisation. In them, we have the beginning of the middle ear; there are a stapes and a fenestra ovalis. Here,also, a larynx appears for the first time; and the shoulder and hip-girdles, and the fore and hind limbs, are developed similarly to those of higher types. The metamorphosis that takes place in the life of frogs and toads is very wonderful. Born almost a lamprey, the frog changes into a creature which is a selachian and something more, for it passes through the further border of the sharks. In its changings, there begin to be formed the rudiments of many important organs which come to perfection in man and his nearest relatives. In them, the upper element of the hyoid arch is modified into the chain of the middle ear, the spicular cartilage into the ring of the ear-parchment, and the skin covering the first cleft into the membrana tympani. The part of the hyoid arch which is developed into the middle ear appears later than that which carries the tongue. The chorda tympani is found for the first time formed by the splitting of the seventh nerve as it arches over the first cleft. The auditory apparatus is quite flat on the face in these animals, and is formed eniirely by the modification of true fish-elements. LECrURE VII. THE reptiles Ae divided into four families, namely, snakes, lizards, turtles, and crocodil.s. They are all similar in having the low-er jaw attached to the skull by a qu,ad-rate bone, whiqlb is not a fixed sus. pensorium, as in the frogs, but forms a true joint. This gives very freeplay and mobility to the jaw. In the snakes, the bones of the skull are extremely hard; but cartilage, when it survives, is perfectly elastic and unossified. The sutures are, as a rule, obliterated at a very early stage.. The skull is largely formed of splint-bones, and the inner parts of the face are also formed of membranous bones, which, however, represent cartilaginous elements in fishes. The outer parts of the face are formed of bones which have no cartilaginous representatives in the lower forms, but exist as splint-bones or dermal scutes in ganoids, osseuts fishes, and amphibia. The whole apparatus of the palate and upper and lower jaws is a very loosely articulated structure, and each bone is inthe form of a simple bar. The hinder part of the endocranium and the auditory capsules are intensely hard, and the orbito-ali-sphenoi.ds are se-parately developed in the membranous walls of the skull. The three bony elements of the auditory capsule remain separate, but thepreotic unites with the basi-sphenoid, the epiotic with the supraoccipital, and the opisthotic with the exoccipital. The only visceral elements that are developed behind the mandible are the rod of the columella and the minute stylo-hyal, which coalesces with the quadrate-The lizards have a movable pin to the lower jaw, like the snakes, but differ from them both externally and internally. One of the most remarkable things in their skull is the retention of two rows of small bones strengthening the eyebrows, and another row over the temporal legion, besides a bone flooring the outer nostril. The parietals are simiiply roof-bones propped up by a slender rod of bone called the columella, which must not be confounded with the bone of the same name in the middle ear. The brain is protected in those animals from injury by the cavity containing it being about three times larger than the brain itself; hence considerable bending of the cranial bones may take place without injury to or pressure upon the brain. Each of theirtail-vertebrie is divided through its centre by a transverse partition. formed of membrane ; and, when the tail is suddenly bent, it snaps off at that point, so in this way the animal escapes from its pursuer when seized upon by that part of the body. The turtle develops in the same mianner as the lizards, but, during its growth, it drops several bodysegments of somatomes. This takes place especially in the neck, and, as a result, it is shortened. The shoulder-girdle and linmbs are not attached to the sternal ribs or sternum, as they do not possess either. The whole of the back skin becomes ossified, so as to forrn a strong. bony box, to which the name carapace is given, and into which the animal can draw itself. The carapace is principally formed by the bony matter from the spines of the back and the rib-shafts spreading. into a thick overlying web of fibrous tissue. The anterior portion of the body is protected by the ossification of the dermis into the plastron. This is formed of four bones on each side, the anterior pair being separated by a single median piece. In the skull, the bones forming the anterior portion are hard and well ossified, while those of the posterior part are only cartilaginous. The pier of the tongue arch or hyoid is a long slender rod, the proximal end of which answers to the stapes, and the rest to the incus; it stretches between the fenestra ovalis and the drum-membrane. NOTE ON LECTURE V.-The proper heterocercal tail is that in whiclh the spine with its lessening joints runs up along the upper lobe; the lower and lesser lobe is fornmed of rays. The homocercal tail (found in most Teleostei) as a rule is heterocercal ; but the many small terminal vertebroe are aborted and form an "urostyle". A diphycercal tail isone with no aborted vertebr-e, but the lobes equal or rounded. GLASGOW.-At Glasgyow, the death-rate in 1878 was 3.6 per I,OOO below the average of the preceding ten years. This improvement was most. marked in the earlier years of life, the death-rate under five years being I IY2 per I,OOO below the average. The meteorological and vital characteristics of 1878 at Glasgow were a high temperature, low rainfall, and a diminished death-rate, especially among children. The geineral death-rate was 25.7 per I,OOO, or the highest of any town in Scotland except Paisley. T he deaths under five years amounted to 48 per cent. of the total deaths ; 87 per cent. of the deaths were certifiel, and 40 per cent. were in friendly societies, on which latter subject Dr. Russell makes some interesting remarks. Of the deaths under one year, onlY 75 per cent. were certified, this percentage being lowered in the case of illegitimate children to 63Y2 per cent. The comparison of I878 with the average of the five preceding years shows an improvement all round, but chiefly in the fatality of infectious diseases. The epidemic of the year was whooping-cough, which alone cauEed 985 deaths. The fevers were never so slightly fatal. Since I869, the death-rate from these diseases has fallen year by year, with a trifling check in 1875. During the last four weeks, only 22 lives bave been lost frolmi small-pox in Glasgow. -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~T HE BRITISH MEDICAL 70URNAL. 396 fMarch 13, 1880-
doi:10.1136/bmj.1.1002.396 fatcat:m2zkj7rv45b3per23qyqtcva7i