NERVOUS CENTRES. (HUMAN ANATOMY. THE SPINAL CORD.) 



6,51 



the centre of which lies the filiform prolon- 

 gation or process of the pia mater. The su- 

 perior limit of the spinal cord is marked by 

 the plane which lies between the occipital 

 foramen and the first vertebra of the neck. A 

 section made in the direction of this plane 

 separates the spinal cord from the medulla 

 oblongata. Immediately above this plane the 

 decussation of fibres of the anterior pyramids 

 takes place, and may be regarded as the na- 

 tural inferior limit to the medulla oblongata. 



Such is the position of the spinal cord in the 

 adult. In the foetus at the third month of 

 intra-uterine life, it occupies the whole spinal 

 canal, and extends quite to the point of the 

 sacrum. At this early period the os coccygis 

 consists of seven vertebrae. Coincident with 

 its reduction to its normal number of segments, 

 is the retraction of the spinal cord within the 

 spinal canal. If the ascent of the cord be ar- 

 rested, the foetus is born with a tail, for the 

 changes of the coccyx become arrested also. It 

 is remarkable that among the inferior animals 

 there is a direct proportion between the length 

 of the spinal cord and that of the tail. The 

 shorter the former, or the higher in the spinal 

 canal it may be, the less will be the latter. In 

 animals with long tails there is no cauda equina, 

 as is the case in the ox, the horse, the squirrel, 

 &c. and the opposite is likewise true, namely, 

 that in animals with a short tail the spinal 

 cord is much shorter and is placed higher up 

 in the spinal canal. In the embryo of the bat, 

 which has a tail, the spinal cord extends down- 

 wards, but when it loses its tail the cord ap- 

 pears to occupy a much smaller portion of the 

 spinal canal. In the tadpole of the frog, like- 

 wise, the spinal cord extends into the tail, but 

 when the tail has disappeared the cord occupies 

 only a portion of the spinal canal.* 



In point of shape, the spinal cord is cylin- 

 droid, slightly flattened on its anterior and pos- 

 terior surfaces, more so on the former than on 

 the latter. At its inferior extremity it gradually 

 tapers to a point. Sometimes, however, we 

 observe a small tubercle immediately above 

 this pointed extremity, situated on the posterior 

 surface. The perfect cylindrical form of the 

 cord is destroyed, not only by this pointed ter- 

 mination and the flattening before and behind, 

 but likewise by a marked change of dimen- 

 sions in certain regions. In the cervical re- 

 gion we observe a distinct swelling or enlarge- 

 ment, which begins a short distance beneath the 

 medulla oblongata, and gradually passes into 

 the dorsal portion, which is the smallest, as 

 well as the most cylindrical part of the cord. 

 This cervical enlargement ( intumescentia cervi- 

 calis) begins opposite the third cervical vertebra, 

 and ends about the third dorsal. The cord con- 

 tinues of a cylindrical form as low as about 

 the ninth or tenth dorsal vertebra, and then 

 passes into the lumbar swelling (intumescentia 

 lumbalis vel cruralis), which occupies a space 

 corresponding to about two vertebrae. This 

 swelling is both shorter and of less diameter 



* Cuvier's Rrport upon Serbs' work, Sur 1'Anat. 

 Temp, du Cerveau. Par. 1824. 



than that in the region of the neck. The inferior 

 extremity of the spinal cord tapers rather sud- 

 denly, and at its point is enclosed in the com- 

 mencement of the filiform prolongation of the 

 pia mater. 



The bulk of the spinal cord is in the direct 

 ratio to that of the body throughout the verte- 

 brate series. And not only is this true with 

 regard to the whole cord, but with respect to 

 its segments. For when any segment supplies 

 nerves to a greater sentient surface, or to more 

 numerous or more powerful muscles than an- 

 other, it exhibits a proportionally greater size. 

 It is thus that we may satisfactorily explain the 

 occurrence of the cervical and lumbar enlarge- 

 ments. Both supply nerves to the extremities, 

 whilst the dorsal portion furnishes them only 

 to the trunk. The upper extremities enjoy, in 

 part, a high degree of tactile sensibility, and 

 they possess great power and extent of mus- 

 cular movement. That portion of the cord 

 therefore from which the nerves to the upper 

 extremities proceed is larger in every way than 

 that which supplies the lower extremities, 

 which, although provided with large and pow- 

 erful muscles, do not enjoy such a range or 

 variety of motion as the upper extremities, nor 

 are they endowed with so exquisite a sensi- 

 bility. 



There are many interesting facts among the 

 lower animals which illustrate and confirm this 

 law. Thus, in animals which have no limbs, 

 as serpents, the cord is of equal size through- 

 out, excepting at its pointed extremity. It is 

 said that in the foetus, before the developement 

 of the limbs, no distinction of size can be dis- 

 covered in the cord, and in persons in whom 

 an arrest in the developement of the upper 

 extremities has taken place, there is no cervical 

 enlargement. Cruveilhier refers to the case of 

 the tortoise as strongly confirmatory of this law. 

 That portion of the spinal cord which corre- 

 sponds to the carapace, which is equally devoid 

 of sensation and motion, is reduced to a mere 

 thread, whilst those segments between which 

 it lies, and from which the nerves of the ex- 

 tremities emanate, are of size duly propor- 

 tionate to their muscular activity and their sen- 

 sibility. In Fishes, the enlargements corre- 

 spond to the fins which are possessed of great- 

 est muscular power. In the gurnard there 

 exist certain very remarkable ganglionic swel- 

 lings, situate on the posterior part of the cer- 

 vical segment of the cord. With these swel- 

 lings nerves are connected, which are distri- 

 buted to organs placed immediately behind the 

 head on the lower part of the body. These 

 organs are endowed with much tactile sensibi- 

 lity, and seem to serve the office of feelers, as 

 the animal gropes along the bottom of the 

 sea. 



The length of the spinal cord in the adult is 

 from sixteen to eighteen inches, according to 

 the statement of Cruveilhier. Its circumfe- 

 rence measures twelve lines at the smallest and 

 eighteen lines at the most voluminous part. 

 Chaussier states that its weight is from the 

 nineteenth to the twenty-fifth part of that of 

 the brain in the adult, and about the fortieth 



