When the congestion is very considerable, it may occasion rupture of bloodvessels and effusion of blood into the sub-amchnoid cavity. This constitutes a form of spinal apoplexy, which is apt to follow concussion of the cord, caused by a fall or by a blow inflicted upon the back. It may follow any of those diseases which are accompanied by convulsions—tetanus, hydro phobia, epilepsy, cerebral apoplexy.
Inflammatory affections of the Membranes, deposits of tubercle or other foreign matter which may cause indumtion of the cord, have their primary seat among the vessels of the pia mater. Inflammation of the membranes is more apt to occur among children than in adults.
Abnormal anatomy of the spinal cord.—The absence of this organ ( amyelia) occurs chiefly in anencepbalous fcetuses. In such cases the posterior wall of the spinal canal is often defi cient, and the canal is occupied by a reddish, vascular pulpy substance. It is a question whether the absence of the cord, in such cases, is to be attributed to a real defect of its deve lopement or to its destruction -while yet in a very delicate semi-fluid state, by the formation of a dropsical effusion, either around it or in the canal or ventricle which exists in it at an early period of its developement. This latter explanation is rendered probable by the fact that all the recorded cases are of fcetuses which had reached an advanced period of intra-uterine developement; and in some of them move ments had been distinctly felt by the mothers, which could not have taken place with a com bined or definite character without the existence of the cord. And in some of the records it is affirmed that the children lived some hours and exhibited movements and even signs of sensa tion, or at least of excitability to stimuli. Such phenomena, if true, leave us no alterna tive but to suppose that the whole cord could not have been absent—some portion must have existed as the centre of these movements, but, being of small size, it escaped the notice .of the observer.
This explanation is likewise confirmed by the occurrence of cases (rare, it is true) in which the brain existed, but the spinal cord was wanting. A very able narrative of a case of this kind has lately been published by Dr. Lonsdale
of Edinburgh. " The anterior and middle lobes of the bmin appeared to be properly developed, and occupied their usual positions in the cranial cavity ; whilst the posterior lobes were much smaller, and were partially squeezed through a large abnormal opening or deficiency in the occipital bone. Ile cerebellum and that part of the occipital bone in which it is normally lodged, were wanting. There was not the slightest vestige of medulla oblongata or spinal cord, and the posterior arches of the vertebrx did not exist."' The fcetus had reached its full term, and its body and limbs were well formed.
An interesting feature which had been well observed in this case, although probably not peculiar to it, but hitherto overlooked, was the relation of the nerves in the cranial and spinal cavities. All the nerves of the medulla oblon gata, and the first, second, and third cervical nerves, hung as loose threads in the cranial cavity or in the upper part of the spinal canal, and presented a looped arrangement, seeming to denote that such is their normal disposition in the nervous centre.
Partial deficiencies of the spinal cord, al though also mre, are more frequent than the total absence of the organ. These occur in connection with other defects of developement. Thus, in spina bifida much of the cord is defi cient, either throughout its entire extent or in those parts where the vertebral wall is defective. In such cases it is probable that the deficiency is attributable to the destructive influence of the dropsical effusion rather than to an original de fect of the organ. In cases in which the upper or the lower extremities have not been deve loped, the usual cervical or lumbar swelling is imperfectly developed, owing to the absence or atrophy of the fibres which would have formed the nerves to those limbs.
Excessive congenital developement of the spinal cord occurs only in those monstrosities which arise from the junction or fusion of the spinal columns of two embryos.
The diseased states of the spinal cord may be enumerated as follows :—hypertrophy, atrophy, induration, softening, suppumtion, deposits of tubercle or of other morbid products.