in the tails Anatomy of the spina arteries; formed by the bony processes of the vertebræ, so that the circulation can never be disturbed in the powerfully muscular tail during any of its movements. b: Fig. 8. The diagram represents a section of the tail of the shark, wherein, besides the spinal canal (a), there is a foramen (b), in the body of the vertebræ (c), for the passage of the aorta, and another for the vena cava.GRANT. The same arrangement is remarkable in the long flexible necks of birds, and in the prehensile tails of monkeys, the continuation of the aorta being in them protected from pressure by the inferior rings of the vertebræ, whilst there are also very numerous inosculations of the arteries of the tail. Here, then, as well as in the human brain and heart, are examples of arteries passing along a course by which they are completely protected from all muscular pressure; whilst we have found other arteries so placed in relation to muscles that they are subjected to muscular compression in order to fulfil a very important function in the animal economy. Anatomy further reveals a circumstance in the circulation. of the spinal chord which is equally essential to be borne in mind, whilst prosecuting our pathological investigations on the nervous system, as that which I have already mentioned in the arterial circulation of the brain; though I am not aware that this important structure has ever been noticed by those ingenious physiological inquirers who have of late years bestowed so much assiduity in investigating the functions of the cerebro-spinal system. The interesting anatomical fact to which I allude is, that the very same physical arrangement which is resorted to by the economy for securing a regular supply of blood to the brain also fulfils a similar office to the spinal chord; and consequently that the spinal chord, like the brain, must be influenced by disturbances in the action of the heart; and hence, too, will arise another assemblage of symptoms in the diseases of the heart, and which I will endeavour to point out are as important an addition to semiology as those derived from the brain itself. are branches tebrals. from the ver It had been shown, and the parts were accurately delineated by Haller, that the whole prolongation of the spinal chord from its exit through the cranium to its termination in the cauda equina, is supplied with blood chiefly by the spinal arteries; the spinal arteries coming off from the vertebrals immediately after these vessels enter the foramen Their anasmagnum of the occipital bone. But the spinal chord, it ought to be recollected, like the brain with its carotids as well as with its vertebrals, has, besides the spinal arteries, small anastomosing branches from the parent trunk, from the intercostal, from the sacral, and from the lumbar vessels; so Fig. 9. that any interruption to the cir- This peculiarity in the ar- posterior spinal arteries. tomosis. Cerebro-spinal system supplied with one channel. blood from inferences. The spinal chord, like the brain, is therefore supplied with blood almost entirely by arteries, the circulation through which can never be interrupted from any movement of the adjacent muscles; whilst at the same time, like the brain, it must be readily influenced by every disorder in the action of the heart. The circumstance of the whole cerebro-spinal system being supplied with blood from the same channels, so that the circulation shall be always modified in both of the great nervous centres at the same moment, and whenever there is any change in the action of the heart, is a fact highly imPhysiological portant to physiology, and will materially assist in explaining the intricate association in the functions of these two subdivisions of the nervous system; whilst it will also elucidate many interesting pathological phenomena. Thus, too, we have obtained an additional proof of the unity of the brain and spinal chord, as one great nervous system; and we can now explain how, in many disturbances of the heart, such as those which accompany febrile diseases, the two nervous centres with the nerves emanating from them are so often simultaneously affected. -Although these peculiarities in the distribution and in the course of the arteries of the cerebro-spinal system have been long known, yet the purposes which they serve in the economy have certainly not been at all understood; nor, indeed, could they have ever been conjectured, far less satisfactorily explained, until the musculo-cardiac function was demonstrated. And it is remarkable that ever since the peculiar course of the arteries of the brain was first described, up to the present period, physiologists should have, one after another, been satisfied with giving nearly the same unphilosophical explanations of the purposes of this singular vascular distribution; some conceiving that the osseous canals through which the vertebral arteries pass merely serve the purpose of a defence, and others conjecturing that this peculiar arrangement was intended to prevent any undue increase in the size of the vessels. Besides these peculiarities in the distribution of the vessels of the brain and spinal chord, all which are intended for securing to those organs a regular supply of the vital fluid, it is also of importance to notice the proportionably large quantity of the mass of the blood which goes to the cerebral system, as this will assist in explaining how that system should be affected, more than any other, when the functions of the heart are disturbed. Brin suplarge propor plied with a tion of blood. calculation. For, if Haller's calculation be correct, or if it be even an Haller's approximation to the truth, that the weight of the brain is about seven pounds in a body weighing two hundred pounds, and that one-sixth of the whole mass of blood goes to the brain, then the brain must be supplied with nearly five times more blood than any of the other parts of the body. Having said thus much on the anatomy of the vertebral arteries, and adverted to the unsatisfactory opinions that have been offered of the purposes of their peculiar course and mode of distribution, it may be fit, also, to draw the attention of the reader to some peculiarities in the carotid arteries; for it must appear strange that, if such decided means were required in order to ensure an uninterrupted circulation in the vertebral, that the carotid arteries should be so much exposed to external injury, and their circulation to so many interruptions from the contractions of the muscles of the neck. Anatomy of the carotid arteries. It seems, however, that the same protection to the circulation, so requisite for the vertebrals, is not wanted in the carotids, although a larger proportion of the blood which goes to the brain is supplied by the internal carotid arteries. The external carotid is distributed upon the neck, the face, and the external coverings of the brain; but the internal carotid conveys to the brain a large tributary stream of the vital fluid; but of so little importance is the supply of blood by the carotids to that of the vertebrals, in animals, that, when a ligature is placed on one of these vessels, no particular symptoms supervene; and even "when both carotids have been tied the animal neither died nor appeared to be very uneasy." Whereas, when the vertebral Haller. G Astley arteries of rabbits are compressed, it is followed by instant death. Before concluding these remarks on the peculiarities in the arteries of the brain, and which I have deemed it essential to bring together in conducting our inquiries on the pathology of the heart, there are likewise some peculiarities in the structure of these vessels, and in their mode of distristructure of. bution after they have entered the cranium, which are here Cerebral arteries Haller. Ratio of the comparative size of the carotid and vertebral arteries in animals. Effects of long necks, worthy of notice. "All the branches of the cerebral arteries contained within the skull have a thin, solid, and more brittle substance than the other arteries of the body;" and as the size of these vessels cannot be much diminished, from the incompressible nature of the cerebral pulp, they do not require to possess those powers of dilatation and contraction like the arteries of the external and soft organs. Comparative anatomy assists us in this, as in most other physiological inquiries; and it appears that, throughout the different races of organized beings, the magnitude of these two sets of vessels-the carotids and the vertebrals-always has a certain ratio with the brain of the class of animals to which they belong. In proportion, therefore, as the anterior or intellectual part of the cerebrum is small, so we find the size of the carotids diminish; and, on the other hand, as the anterior portion increases, so the carotids become large. And it has been made a general remark," observes Bichat, "that animals with long necks, in whom the brain is necessarily far from the heart, the heart cannot excite the brain so vigorously; their intelligence is more limited, consequently the cerebral functions are more narrowed. On the contrary, those animals who have short necks, and consequently in whom the brain and heart more closely approach, commonly have a corresponding mental energy. Mankind, who have the head placed far above the shoulders, compared with those in whom it is very close, sometimes offer the same phenomena." "A Roman Emperor, to show the certainty of his hand, |