Page 407 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Nervous system (systema nervosum) 389

it is able – once fully myelinated – to conduct afferent by venous networks. The veins and connective tissue
VetBooks.ir impulses and pass them on to the axon. Both the peripheral compartments facilitate the resorption of fluid. This is of
and central branches thus act as fully functional axons (see clinical significance in epidural anaesthesia.
Chapter 5, ‘Nervous tissue’).
Pseudo-unipolar neurons within spinal ganglia are Leptomeninges
sometimes accompanied by multipolar nerve cells and The leptomeninges consist of a thin outer membrane,
other small neurons; these may be interneurons or cells the arachnoid, and an inner layer, the pia mater, that lies
of the autonomic nervous system. Spinal ganglia are sub- directly adjacent to the underlying brain and spinal cord
divided by sheets of loose connective tissue (endoneurium) (see Veterinary Anatomy of Domestic Animals: Textbook and
containing delicate capillary networks. Colour Atlas). The arachnoid (arachnoidea encephali and
Most sensory ganglia of the cranial nerves are similar spinalis) is a thin, avascular connective tissue layer that
in structure to the spinal ganglia. The spiral (cochlear) lies against the internal surface of the dura mater. Sheets
ganglion and vestibular ganglion associated with the vesti- of flattened cells create a mesothelium-like lining of the
bulocochlear nerve are the only sensory ganglia in which cavum subdurale (the potential space between the dura
the neurons are bipolar. mater and arachnoid). This cell layer, termed the subdural
neurothelium, plays an important role in fluid resorption.
Autonomic ganglia In contrast to the dura mater, the arachnoid surrounding
The nerve cells found in ganglia of the autonomic nervous the brain features avascular villous projections, arach-
system (ganglia autonomica) are multipolar (neurona noid granulation villi (granulationes arachnoideales).
multipolaria) (Figure 17.13). These variably sized cells are These are presumed to participate in metabolic exchange
surrounded by satellite cells (amphicytes, gliocyti ganglii). between venous blood and cerebrospinal fluid.
Autonomic ganglia are permeated by connective tissue Arachnoid trabeculae (trabecula arachnoidea), com-
septa that give rise to small compartments. posed of collagen fibres, traverse the subarachnoid space
Autonomic ganglia include segmentally positioned (cisterna subarachnoidea) between the arachnoid and the
swellings within the sympathetic trunk, prevertebral pia mater. This fibrous network has a mesothelial-like
sympathetic ganglia (e.g. coeliac ganglion) and parasym- lining resembling the subdural neurothelium. The suba-
pathetic ganglia (e.g. ciliary ganglion). rachnoid space is filled with cerebrospinal fluid (liquor
cerebrospinalis). The pia mater of the brain and spinal
Meninges cord (pia mater encephali and spinalis) is an extensively
The central nervous system is encased in connective tis- vascularised layer composed principally of loose connec-
sue sheaths referred to as the meninges (Figure 17.15). The tive tissue. It lies loosely against the external surface of the
meninges are composed of the: central nervous tissue as a single-celled mesothelium, in
direct contact with the superficial glial limiting membrane.
· pachymeninx (dura mater), Up to a certain depth, the pia mater extends into the fis-
· leptomeninges, comprising the sures on the surface of the central nervous system. Blood
− arachnoid and vessels accompany the connective tissue, giving rise to a
− pia mater. perivascular space between the blood vessels and the pia
mater. The blood vessels continue into the nervous tissue
Pachymeninx (dura mater) where, in conjunction with the glial cells, they form the
The outermost meningeal layer, the dura mater, con- blood–brain barrier. This functional barrier to penetra-
sists of a taut meshwork of connective tissue containing tion of blood solutes is composed of modified endothelial
few vessels. In the horse, the dura mater covering the cells lining the capillaries and the glial limiting membrane.
cerebrum (dura mater encephali, cranial dura mater) is
extensively fused with the cranium. This phenomenon is Ventricles (ventriculi)
limited to bony protuberances in other domestic mam- The central nervous system encloses a continuous fluid-
mals. The dura mater encephali contains venous sinuses filled system comprising a series of chambers and a central
lined with epithelium (sinus venosi). Externally, these are canal. The walls of these cavities are lined by ependymal
bounded by fibrous layers of the dura. The sinuses do not cells. This epithelium-like layer is considered part of the
contain valves. neuroglia and is described in Chapter 5, ‘Nervous tissue’.
The dura mater surrounding the spinal cord (dura A special feature of the ventricles of the brain is the
mater spinalis, spinal dura mater) forms a large tube that choroid plexus (plexus choroideus). Located in circum-
is separated from the periosteum of the vertebral canal scribed regions of the walls of ventricles, the choroid
(endorhachis) by loose connective tissue interspersed with plexuses consist of pial connective tissue and numerous
fat. This epidural space (spatium epidurale) is permeated branched, thin-walled vessels. The surface of these vascular

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