Foramen magnum

“ The single large temporal opening is bounded below by the postorbitals and squamosal, above by the parietals, which are more or less elevated in the middle in a crest. There is a pineal foramen. The squamosals, in some at least, join each other back of the parietals on the upper surface of the skull. There are no interparietals, tabulars, or supratemporals.
On the occipital surface the supraoccipital is excavated more or less by the foramen magnum, which may extend to the roof, dividing the bone. The paroccipitals are always fused with the exoccipitals. ”

“ Lumbar puncture, a method that presents some danger when the surgeon has to deal with a subtentorial tumour, but which bears in its train excellent results from the point of view of reduction of intracranial pressure. The danger arises from the fact that the sudden escape of cerebro-spinal fluid may cause the brain-stem to be engaged in the foramen magnum, with disastrous results on the medullary centres. ”

“ The rapid withdrawal of cerebro-spinal fluid has been [228] followed by disastrous results, the medullary stem being suddenly forced downwards into the foramen magnum, with corking up of that aperture, and complete upset of the medullary centres. Such a disaster is the more likely to occur when the tumour is subtentorial in position. Looking at the question from the broadest point of view, it would appear that the performance of lumbar puncture in cases of suspected brain tumour is fraught with considerable peril.
X-ray examination. X-ray examination should always be carried out. ”

“ It is true that there is always a possibility of setting up a septic meningitis by lumbar puncture, but this is a very remote possibility and with any reasonable care it is not to be considered. Lumbar puncture also has a considerable danger in cases of increased intracranial pressure. In cases of brain tumor where the tumor is located in the posterior fossa, sudden death may occur from withdrawal of spinal fluid. This is supposed to be due to the medulla being pressed down into the foramen magnum and causing paralysis of respiration. ”

“ If death was not too soon the consequence of the puncture of the head of hydrocephalic patients, I am persuaded that we should see the bones gradually contract, and restore the cavity of the cranium to its natural dimensions. When we remove the dead piece from a long bone in necrosis, the new bone, formed on the exterior by means of the periosteum contracts in an evident manner. In paralysis of the optic nerve, its foramen becomes narrower. The orbit contracts when a cancerous eye has been extirpated. ”

“ Gauze plugs may usually be relied on to stop the bleeding, but they possess the great disadvantage of exercising pressure on the cortex and preventing expansion of the brain.
Occlusion of the foramen spinosum and ligature of the carotid arteries complicate and prolong the operation. The foramen spinosum is occluded in the following manner: the dura mater is quickly stripped up with the fingers away from the base of the skull, the soft parts gently but firmly retracted and the foramen identified. ”

“ The borders of the foramen are hardly as smooth as they would be had they transmitted some erratic tendon or artery, and outside of its borders there is no trace of any groove such as a large artery or a tendon would produce. As to tumor, there is very little if any excavation or absorption of the walls in the opening, and no thickening of the bone.
The formation must have taken place very early in the life of the individual, as the form of the entire head of the ulna is affected. In all probability the anomaly of the foramen is due to congenital causes. ”

“ A curved incision is made through the skin and subcutaneous tissues parallel to and below the infra-orbital margin, the tissues retracted, and the small muscles overlying the infra-orbital foramen divided or retracted so [309] as to allow exposure of the foramen. The nerve is isolated and divided. The peripheral end is seized with forceps and avulsed. The central end may be treated after similar fashion, or, preferably, again exposed in the infra-orbital groove or canal, the tissues being peeled away from the floor of the orbit and the nerve sought for as it traverses the infra-orbital canal. ”

“ In figure 109, I have placed a diagram of the base of the skull of Pyrotherium, along beside that of Palaeomastodon, for comparison of the basicranial foramena. The skull of Palaeomastodon is somewhat more elongated, especially in the posterior part. In both, there are two antorbital foramena; the post-palatal foramena of Pyrotherium are a trifle further back, but this palatal region in both is of the same type which is peculiar to elephants and Pyrotherium. In Pyrotherium the condylar foramen is separate, while in elephants it is fused in with the foramen lacerum posterior. ”

“ The sculpturing of the internal surface of the cranial cavity bespeaks the high vascularity of the soft parts which it contained, and there are evident indications that the upper and lateral surfaces of the brain had been disposed in a few simple parallel longitudinal convolutions. The two anterior condyloid foramina (m) have the same relative position as the single corresponding foramen in the Glossothere, Orycterope, and Armadillos, and the inner surface of the skull slopes outwards from these foramina to the inner margin of the occipital condyle. ”

“ General thinning of intervertebral substance through a condition of disturbed metabolism itself produced through the agency of some one serious subluxation, may narrow all the foramina and increase impingement of nerves at any point where a slight subluxation previously existed. An irritated nerve may become swollen and the nerve impinged at the foramen. ”

“ The dorsal margin of the groove projects mediad in the form of a sharp ridge which is broadest caudad, where it often reaches nearly to the posterior clinoid process. The groove passes craniad into three foramina. The first (cranial) of these, the orbital fissure (i) , is large and lies between the wing, the body, and the pterygoid process. It is incomplete, but is completed by the presphenoid. The second foramen is small and rounded; it is the foramen rotundum (j) . The third, foramen ovale (k) , is larger and oval and penetrates the wing through about the middle of its longitudinal axis. ”

“ It passes craniad and dorsad at the side of the thoracic portion of the longus colli muscle, and enters the foramen transversarium of the sixth cervical vertebra. It passes thence craniad through the foramina transversaria (which together form the vertebrarterial canal) and gives off at the intervertebral foramina branches to the muscles of the neck and branches which pass across the ventral surface of the spinal cord to join A. spinalis anterior. Craniad of the foramen transversarium of the atlas the vertebral artery turns dorsad in the groove on the lateral surface of the atlas. ”

“ The anterior region, as is the laminal surface, is covered with foraminæ; in this case more minute. In the recent state it is covered by the sensitive sole. The posterior region, lying immediately behind the semilunar crest, shows on each side of a median process a large foramen, the Plantar Foramen. From this foramen runs the Plantar Groove, a channel, bounded above by the superior edge, and below by the semilunar crest of the bone, which conducts the plantar arteries into the Semilunar Sinus, a well-marked cavity in the interior of the bone. ”

“ That portion of the bone lying laterad of a line joining the medial borders of these two pairs of foramina is known as the lateral mass of the sacrum and is composed of the fused transverse processes of the sacral vertebræ. At the caudal margin of the ventral surface there is a notch between the lateral mass and the centrum (g) . When the caudal vertebræ are articulated, this notch helps to form a foramen for the third sacral nerve.
The dorsal surface (Fig. 6) is narrower at its cranial end than is the ventral surface. ”

“ Also diagnosed by a unique combination of the following features: five maxillary foramina consisting of four scattered small ones anteroposteriorly arranged in a row and a large one close to the ventral extremity of the jugal articular surface of the maxilla; markedly convex dorsally maxillary and dentary occlusal surfaces; elevated maxillary body that is approximately 150% longer than tall; laterally exposed, large anterior foramen limited to the anterior half of the anterodorsal surface of the maxilla and located just lateral to the premaxillary articular surface of the bone ”

“ About two-thirds of the way back the sagittal suture becomes less serrated and on each side of it the small parietal foramen may be seen. This only transmits a small emissary vein (see Veins) in the adult, but, as will be seen later, is of considerable morphological interest. As middle life is reached the cranial sutures tend to become obliterated and the bones can no longer be separated; this fusion begins at the places, where the sutures are least deeply serrated, and as a rule the sagittal suture disappears between the two parietal foramina between thirty and forty years of age. ”

“ To reach the foramen rotundum (second division) —a rather more difficult procedure—it is necessary to find the posterior border of the orbital process of the malar bone, prolonging this line downwards to the lower border of the zygoma and inserting the needle 1⁄2 cm. posterior to this point. The needle is pushed horizontally inwards and the point directed slightly upwards, the foramen being reached in the pterygo-maxillary fossa at a depth of about 3 cm. from the zygoma. ”

“ Rami musculares.—Muscular branches are given off near the supracondyloid foramen to the biceps (4) , epitrochlearis (6) , and brachialis muscles. A nutrient artery leaves [297] the brachial proximad of the supracondyloid foramen, either separately or in common with the muscular branches, and passes into the nutrient foramen at the junction of the middle and distal thirds of the shaft of the humerus, supplying the bone. ”

“ It happens very often, that the occipital artery seems to send a branch through the mastoid foramen; it is very commonly believed that this branch appertains to the dura mater, and is an accessory nutritive artery (Art. meningea externa accessoria) . Now, when a series of isolated injections of the occipital artery are made, it is easy to show that, in many instances, the artery which passes into the mastoid foramen does not pass through it, and that it is, therefore, no meningeal artery. ”

“ As the ductus venosus and hypogastric arteries terminate in blind ends and become useless as soon as the umbilical cord is cut, they soon begin to atrophy and are obliterated within a few days after birth. This means that less blood is poured into the right auricle, which naturally results in relatively less tension in the right heart and an increased pressure in the left, which tends to close the foramen ovale. The foramen ovale does not entirely disappear at once, however, but closes gradually, sometimes remaining open for months. ”

“ It consists of two layers, an outer fibrous and an inner cellular layer; the cells, which are called osteoblasts, are continuous with those lining the Haversian canals and the medullary cavity.
The arrangement of the blood vessels determines to some extent the incidence of disease in bone. The nutrient artery, after entering the medullary canal through a special foramen in the cortex, bifurcates, and one main division runs towards each of the extremities, and terminates at the ossifying junction in a series of capillary loops projected against the epiphysial cartilage. ”

“ Its occlusion may be effected by means of a sterilized wooden match or bone peg, by a small piece of bone derived from those osseous fragments which have been removed during the operation, or with [147] the aid of a piece of catgut; in each case the occluding medium should be well driven into the foramen. ”

“ Six per cent. exhibit an anomaly which is extremely rare among normal individuals—the olecranon foramen, a perforation in the head of the humerus where it articulates with the ulna. This is normal in the ape and dog and is frequently found in the bones of prehistoric man and in some of the existing inferior races of mankind. ”

“ These three nerves are, however, so protected by their dural sheaths that they generally escape injury. [21]
In the following cases the nerves were involved:—
The patient was admitted suffering from a fracture resulting from a blow on the posterior parietal region. During the next four days no special symptoms developed. On the fifth day, during a sudden attack of dyspnœa and dysphagia, death occurred. A fissured [107] fracture was found which extended into the jugular foramen, a region occupied by blood-clot.
A man committed suicide by means of a pistol-bullet fired through the mouth. ”

“ In such conditions it is necessary that a comminuted or fissured fracture of the vault should be present, that the underlying dura mater should be torn, and that the adjacent brain substance should be severely contused or lacerated. Cerebro-spinal fluid may alone escape through the gap in the skull to the subaponeurotic and subcutaneous regions, leading to the formation of a false traumatic meningocele. When the brain shares in the outward protrusion the condition is known as false traumatic meningo-encephalocele. ”

“ The nerve-trunks may be contused or torn across, especially in basal fractures which traverse their foramina of exit; blood may be effused into their sheaths as a result of injuries not attended with fracture; or they may be pressed upon by an inflammatory effusion, a tumour, a gumma, or an aneurysm invading the base of the skull. When the nerve is merely contused, or pressed upon by blood-clot, the paralysis tends to pass off in the course of a few days. ”