being inhibited or exalted, has been well shown in the case of the vaso-motor centre, before described. This power, which can be exerted from the periphery, is very important in regulating the action even of partially automatic centres such as the respiratory centre.

CHAPTER XVIII.

THE CEREBRO-SPINAL NERVOUS SYSTEM.

The physiology of the cerebro-spinal nervous system includes that of the Spinal Cord, Medulla Oblongata, and Brain, of the several Nerves given off from each, and of the Ganglia on those nerves.

Membranes of the Brain and Spinal Cord.-The Brain and Spinal Cord are enveloped in three membranes (1) the Dura Mater, (2) the Arachnoid, (3) the Pia Mater.

(1.) The Dura Mater, or external covering, is a tough membrane composed of bundles of connective tissue which cross at various angles, and in whose interstices branched connective-tissue corpuscles lie: it is lined by a thin elastic membrane, and on the inner surface, and, where it is not adherent to the bone, on the outer surface also is a layer of endothelial cells very similar to those found in serous membranes. (2.) The Arachnoid is a much more delicate membrane, very similar in structure to the dura mater, and lined on its outer or free surface by an endothelial membrane. (3.) The Pia Mater consists of two chief layers, between which numerous blood-vessels ramify. Between the arachnoid and pia mater is a network of fibrous-tissue trabeculæ sheathed with endothelial cells: these sub-arachnoid trabeculæ divide up the sub-arachnoid space into a number of irregular sinuses. There are some similar trabeculæ, but much fewer in number, traversing the sub-dural space, i.e., the space between the dura mater and arachnoid.

Pacchionian bodies are growths from the sub-arachnoid network of connective-tissue trabecula which project through small holes in the inner layers of the dura mater into the venous sinuses of that membrane. The venous sinuses of the dura mater have been injected from the sub-arachnoidal space through the intermediation of these villous outgrowths.

A. The Spinal Cord and its Nerves.

The Spinal cord is a cylindriform column of nerve-substance connected above with the brain through the medium of the me dulla oblongata, and terminating below, about the lower border of the first lumbar vertebra, in a slender filament of grey substance, the filum terminale, which lies in the midst of the roots of many nerves forming the cauda equina.

Structure. The cord is composed of white and grey nervous substance, of which the former is situated externally, and constitutes its chief portion, while the latter occupies its central or axial portion,

and is so arranged, that on the surface of a transverse section of the cord it appears like two somewhat crescentic masses connected together by a narrower portion or isthmus (fig. 330).

Passing through the centre of this isthmus in a longitudinal direction is a minute canal (central canal), which is continued through the whole length of the cord, and opens above into the space at the back of medulla oblongata and pons Varolii, called the fourth ventricle. It is lined by a layer of columnar ciliated epithelium. The spinal cord consists of two exactly symmetrical halves,

rately on the wall of the skull and on the several vertebræ natural exit from the cranio-spinal cavity. (After Bourgery.)

separated anteriorly and posteriorly by vertical fissures (the posterior fissure being deeper, but less wide and distinct than the anterior), and united in the middle by nervous matter which is usually described as forming two commissures-an anterior commissure, in front of the central canal, consisting of medullated nerve-fibres, and a posterior commissure behind the central canal consisting also of medullated nerve-fibres, but with more neuroglia,

Fig. 330.-Different views of a portion of the spinal cord from the cervical region, with the roots of the nerves (slightly enlarged). In A, the anterior surface of the specimen is shown; the anterior nerve-root of its right side being divided; in в, a view of the right side is given; in c, the upper surface is shown; in D, the nerve-roots and ganglion are shown from below. 1. The anterior median fissure; 2, posterior median fissure; 3, anterior lateral depression, over which the anterior nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink; 5, anterior roots passing the ganglion; 5', in A, the anterior root divided; 6, the posterior roots, the fibres of which pass into the ganglion 6'; 7, the united or compound nerve; 7', the posterior primary branch, seen in A and D to be derived in part from the anterior and in part from the posterior root. (Allen Thomson.)

which gives the grey aspect to this commissure (fig. 330, B). Each half of the spinal cord is marked on the sides (obscurely at the lower part, but distinctly above) by two longitudinal furrows, which divide it into three portions, columns, or tracts, an anterior, lateral, and posterior. From the groove between the anterior and lateral columns spring the anterior roots of the spinal nerves (B and c, 5); and just in front of the groove between the lateral and posterior column arise the posterior roots of the same (B, 6): a pair of roots on each side corresponding to each vertebra (fig. 329).

up

of

White matter.-The white matter of the cord is made medullated nerve-fibres, of various sizes, arranged longitudinally around the cord under the pia mater and passing in to support the individual fibres in the delicate connective tissue or neuroglia made up of a very fine reticulum, with both small cells almost filled up by nuclei and stellate branching corpuscles.

The general rule respecting the size of different parts of the cord appears to be, that the size of each part bears a direct proportion to the size and number of nerve-roots given off from

Fig. 331.-Section of grey matter of anterior cornu of a calf's spinal cord; nf, nerve-fibres of white matter in transverse section, showing axis-cylinder in centre of each; a r, anterior roots of spinal nerve passing out through white matter; g c, large stellate nerve-cells with nuclei; they are seen imbedded in neuroglia. (Schofield.)

itself, and has but little relation to the size or number of those given off below it. Thus the cord is very large in the middle and lower part of its cervical portion, whence arise the large nerve-roots for the formation of the brachial plexuses and the supply of the upper extremities, and again enlarges at the lowest part of its dorsal portion and the upper part of its lumbar, at the origins of the large nerves which, after forming the lumbar and sacral plexuses, are distributed to the lower extremities. The chief cause of the greater size at these parts of the spinal cord is increase in the quantity of grey matter; for there seems reason to believe that the white or fibrous part of the cord becomes gradually and progressively larger from below upwards, doubtless

from the addition of a certain number of upward passing fibres

from each pair of nerves.

From careful estimates of the number of nerve-fibres in a transverse section of the cord towards its upper end, and the number entering it by the anterior and posterior roots of each pair of nerves, it has been shown

Grey matter. The grey matter of the cord consists essentially of an extremely delicate network of the primitive fibrillæ of axis-cylinders, and which are derived from the ramification of multipolar ganglion cells of very large size, containing large round. nuclei with nucleoli. This fine plexus is called Gerlach's network, and is mingled with the meshes of neuroglia, which in some parts is chiefly fibrillated, in others mainly granular and punctiform. The neuroglia is prolonged from the surface into the tip of the posterior cornu of grey matter and forms a jelly-like transparent substance, which when hardened is found to be reticular, and is called the substantia gelatinosa of Rolando.

Fig. 332.-Transverse section of half the spinal cord in the lumbar enlargement (semi-diagrammatic). 1. Anterior median fissure; 2, posterior median fissure; 3, central canal lined with epithelium; 4, posterior commissure; 5, anterior commissure; 6, posterior column; 7, lateral column; 8, anterior column. The white substance is traversed by radiating trabeculae of pia mater. 9, Fasiculus of posterior nerve-root entering in one bundle; 10, fasciculi of anterior roots entering in four spreading bundles of fibres; b, in the cervix cornu, decussating fibres from the nerve-roots and posterior commissure; c, posterior vesicular columns. About half way between the central canal and 7 are seen the group of nerve-cells forming the tractus intermediolateralis; e,e, fibres of anterior roots; e, fibres of anterior roots which decussate in anterior commissure. (Allen Thomson.) x 6.

The multipolar cells are either scattered singly or arranged in groups, of which the following are to be distinguished on either