Frederick Gymer Parsons

“ In the lower vertebrates as well as in the embryo of man, a number of cartilaginous or bony arches encircle the mouth and pharynx (anterior part of the food tube) , just as hoops encircle a barrel. There is little doubt that, when they first appeared in the history of evolution, all these bars supported gills and bounded gill slits, but in all existing types the first arch has been modified to surround the mouth and to act as both upper and lower jaws, gaining in different animals a more or less complete connexion with the cranium or brain-containing part of the skull. ”

“ In the intersegmental tissue ventral to each of the intervertebral disks a transverse rod of cells, known as a hypochordal bar, is formed which connects the heads of two opposite ribs. In man the greater number of these either disappear or form the middle fasciculus of the stellate ligament which joins the head of the rib to the intervertebral disk, but in the case of the atlas the rod chondrifies to form the anterior (ventral) arch which is therefore intersegmental, while the segmental body of the atlas, through which the notochord is passing, joins the axis to form the odontoid process. ”

“ Hence the sacrum and coccyx of the adult are the false, whilst the lumbar, dorsal and cervical are the true vertebrae.
The vertebrae are irregularly-shaped bones, but as a rule have certain characters in common. Each possesses a body and an arch, which enclose a ring, with certain processes and notches. The body, or centrum, is a short cylinder, which by its upper and lower surfaces is connected by means of fibro- cartilage with the bodies of the verte- brae immediately above and below. The collective series of vertebral bodies forms the great column of the spine. ”

“ It is the decidua basalis which is specially interesting in considering the formation of the placenta. That part which is nearest the ovum is called the "stratum compactum" but farther away the uterine glands dilate and give a spongy appearance to the mucous membrane which earns this particular layer the name of "stratum spongiosum." Processes grow out from the surface of the ovum which penetrate the stratum compactum of the decidua basalis and capsularis and push their way into the enlarged maternal blood sinuses; these are the "chorionic villi." ”

“ The Spine, Spinal or Vertebral Column, chine or backbone in man consists of a number of superimposed bones which are named vertebrae, because they can move or turn somewhat on Spine
each other. It lies in the middle of the back of the neck and trunk; has the cranium at its summit; the ribs at its sides, which in their turn support the upper limbs ; whilst the pelvis, with the lower limbs, is jointed to its lower end. ”

“ The upper limb in man may be subdivided into a proximal part or shoulder, a distal part or hand, and an intermediate shaft, which consists of an upper arm or brachium, and a forearm or ante-brachium. In each of these subdivisions certain bones are found : in the shoulder, the clavicle and scapula ; upper arm, the humerus; in the forearm, the radius and ulna, the bone of the upper arm in man being longer than the bones
of the forearm ; in the hand, the car- pal and metacar- pal bones and the phalanges. ”

“ It is in the deeper layers of these cells that the pigment of the negro's skin is found. The fifth and deepest layer of the epidermis is the stratum germinativum, in which there is only one layer of columnar cells. The whole of the epidermis is non-vascular, and it will be noticed that as the different layers approach the surface the cells become more and more flattened. The true skin, dermis or corium is composed of a felted network of white fibrous tissue with a small number of yellow elastic fibres interspersed. ”

“ Owing to the inclination of the pelvis, the base of the sacrum is nearly 4 in higher than the upper border of the pubic symphysis. The female pelvis is distinguished from the male by certain sexual characters. The bones are more slender, the ridges and processes for muscular attachment more feeble, the breadth and capacity greater, the depth less, giving the greater breadth to the hips of a woman ; the inlet more nearly circular, the pubic arch wider, the distance between the tuberosities greater, and the aceta- bulum smaller in the female than in the male. ”

“ In the infant at the time of birth the sacro- coccygeal part of the spine is concave forward, but the rest of the spine, except a slight forward concavity in the series of thoracic vertebrae, is almost straight. When the infant begins to sit up in the arms of its nurse, a convexity forward in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forward in the region of the loins. Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are associated with the erect attitude of man. ”

“ The Amphibia have a definite compact pancreas which lies in the U-shaped loop between the stomach and duodenum, and is massed round the bile duct. In the Reptilia there are sometimes several ducts, as in the crocodile and the water tortoise (Emys) , and this arrangement is also found in birds (the pigeon, for instance, has three ducts opening into the duodenum at very different levels) . In mammals the gland is usually compact, though sometimes, as in the rabbit, it is diffuse. It usually has two ducts, as in man, though in many animals, such as the ox, sheep and goat, only one persists. ”

“ As appendages of the skin are found the hairs, the nails and the sebaceous and sweat glands.
Hair.—The hairs are found in man on the scalp, eyelids, eyebrows, armpits, pubic region, vestibule of the nose, external auditory meatus, face, ventral surface of the trunk and dorsal surfaces of the leg, forearm and hand; indeed the only places which are quite free from them are the palms of the hands, soles of the feet and the glans penis. In some places, such as the armpits, pubic region and the face of the male they grow to a considerable length at and after puberty. ”

“ There are several cervical vertebrae instead of one, which is all the amphibians have. The odontoid bone is usually separate both from the atlas and axis while, between the atlas and the skull, there are rudiments of an extra intervertebral dorsal structure or pro-atlas in some forms such as the crocodile and Sphenodon lizard. Two sacral vertebrae (i.e. vertebrae articulating with the ilium) are generally present instead of the one of the Amphibia, but they are not fused together as in mammals. ”

“ The shaft of the bone is three-sided; its inner
surface is subcutaneous, and forms the shin ; its outer and posterior surfaces are for the origin of muscles; the anterior border forms the sharp ridge of the shin, and terminates superiorly in a tubercle for the insertion of the extensor tendon of the leg; the outer border of the bone gives attachment to the interosseous membrane of the leg. ”

“ Its costal or ventral surface is in relation to the ribs, from which it is separated by certain muscles: one, called sub- scapularis, arises from the surface itself, which is often termed subscapular fossa. The dorsum or back of the scapula is traversed from behind forward by a prominent spine, which lies in the proper axis of the scapula, and subdivides this aspect of the bone into a surface above the spine, the supraspinous fossa, and one below the spine, the infra-spinous fossa. ”

“ In many of the lower mammals (e.g. Ungulata and Carnivora) the hyoid arch is much more completely ossified than it is in man, tympano-, stylo-, epi-. cerato- and basihyal elements all being bony (see fig. 34) . It is of interest to notice that in the hares and rabbits the body of the hyoid has occasionally been found in two pieces, indicating its derivation from the second and third visceral arches. The fourth and fifth arches, which form the thyroid cartilage in mammals, are considered in the article Respiratory System. ”

“ The os innominatum has three primary centres for the ilium, ischium and pubis.
The special centres for the crest of the ilium are probably a serial repetition of those for the vertebral border of the scapula (see fig. 19) ; that for the anterior inferior spine is a purely human traction epiphysis connected with the use of the straight head of the rectus femoris in the upright position. The centre for the pubic symphysis probably represents the epipubis of amphibians, while that for the tuberosity of the ischium is the hypoischium of reptiles (see sub- section on comparative anatomy) . ”

“ Ribs are present in the cervical, thoracic and lumbar regions, and in the Chelonia (tortoises) the cervical ones blend with the vertebrae as they do in higher forms. In crocodiles a definite vertebrarterial canal is established in the cervical region which henceforward becomes permanent. The shafts of the ribs are sometimes all in one piece as in snakes or they may be developed by three separate centres as in Sphenodon with intervening joints. In these cases dorsal, intermediate and ventral elements to each shaft are present. ”

“ The caudal vertebrae are fused into a flattened bone, the pygostyle, to support the tail feathers. In the fossil bird Archaeopteryx the centra are amphicoelous and the long tail has separate caudal vertebrae. The ribs are few and consist of dorsal (vertebral) and ventral (sternal) parts; the former almost always have uncinate processes. Free cervical ribs are often present and Archaeopteryx possessed abdominal ribs. The sternum is very large and in flying birds (Carinatae) has a median keel (carina) projecting from it, while the non-flying, ostrich-like birds (Ratitae) have no such structure. ”

“ Sudoriparous or sweat glands (see fig. 2) are found all over the surface of the body, but are specially numerous on the palms and soles. It is esti- mated that in the palm there are nearly 3000 to a square inch, while in the skin of the back they do not reach 500 to the same area. In the armpits and groins they are very large. Each consists of a single long tube, lined by columnar epithelium, and coiled up into a ball or glomerulus in the subcutaneous tissue, after which it pierces the corium and epidermis to reach the surface at the porus sudorijerus. ”

“ In the centre of each hair is the medulla or pith, though this is not always present; it is composed of nucleated cells containing pigment, fat and air spaces. Outside this is the fibrous layer or cortex, also containing pigment and air spaces, while most superficially is the cuticle made up of overlapping scales. The hair grows at its root from a hair follicle (see fig. 2) , which is a tubular inpushing of the epidermis into the true skin or, in the case of large hairs, deeper still into the superficial fascia. ”

“ The colour of the human skin has always held an important place among physical criteria of race. Physiology explains colour as a consequence of climate and even diet. The pigment or colour- ing matter under the epidermis, or rather under the second or Malpighian skin, is not peculiar to the Negroid and other coloured races, but is common to all human beings. ”

“ The mandible no longer articulates with the quadrate but forms a new articulation, by means of the con- dyle, with the glenoid cavity of the squamosal, and many modern morphologists, including the writer, are inclined to agree with H. Gadow that the quadrate has probably become the tympanic bone. In many mammals (e.g. Carnivora) this bone swells out to form the bulla tympani. The derivation of the auditory ossicles has been discussed in the section on embryology as well as in the article Ear. The presence of a chain of ossicles is peculiar to the Mammalia. ”

“ The human sacrum is broader in proportion to its length than in other mammals ; this great breadth gives solidity to the lower part of the spine, and, conjoined with the sjze of the lateral articular surfaces, it permits a more perfect junction with the haunch-bones, and is correlated with the erect position. Owing to the need in woman for a wide pelvis, the sacrum is broader than in man. ”

“ It is more difficult to trace the fate of these structures in existing vertebrates than it is with the carpal bones. In man the astragalus probably contains the tibiale, tibiale marginale and intermedium, the latter structure possibly accounting for the occasional os tri- gonum, already mentioned in the subsection on embryology. The fibulare and fibulare marginale probably form the calcaneum, though it is unlikely that the epiphysis at the back of that bone represents any integral part of a generalized tarsus. ”

“ The lower ramus is really more horizontal than the upper (which used to be called the horizontal ramus) , and runs backward and outward from the body to meet the ramus of the ischium and so form the subpubic arch.
The ischium (fig. 16) , like the ilium and pubis, has the fundamental form of a three-sided prismatic ischium. rod. One extremity (the upper) completes the acetabulum, whilst the lower forms the large V. prominence, or tuber ischii. The surfaces of the bone are internal or pelvic, antero-external, and postero- external. The pelvic and postero
the innominate bone. ”

“ In the Cyclostomata a reduction of the number of gill slits takes place, and an increased area for respiration is provided by the gill pouches lined by pleated folds of entodermal mucous membrane; these form the simplest type of true internal gills. In the larval lamprey (Ammocoetes) there are eight gill slits opening from the pharynx, but in the adult (Petromyzon) they are reduced to seven, and a septum grows forward separating the ventral or bronchial part of the pharynx from the dorsal or digestive part. ”