BLOOD-VESSELS. clxv 



the principle of formation of nerve-fibres advanced by von Hensen is substantially the 

 same as that previously published by Dr. Beale. 



Re-union and regeneration of nerves. The divided ends of a nerve that has been 

 cut across readily reunite, and in process of time true nerve-fibres are formed in the 

 cicatrix, and restore the continuity of the nervous structure. The conducting pro- 

 perty of the nerve, as regards both motion and sensation, is eventually re-established 

 through the re-united part. But, immediately after the section, a process of degene- 

 ration begins in the peripheral or severed portion of the nerve. The medulla of the 

 white fibres degenerates into a granular mass consisting, apparently, of fatty mole- 

 cules, and is then totally removed, while the axial fibre, with the primitive sheath 

 and nuclei, remains ; but, according to some authorities, the latter also suffer more or 

 less. After reunion takes place, and usually not till then, the medulla is gradually 

 restored, the restoration proceeding from the point of reunion outwards along the 

 nerve, which is then restored to its primitive integrity both in structure and function. 

 From experiments of Philipeaux and Vulpian, it would seem that in very young 

 animals restoration of the severed portion may take place without previous reunion. 



The degeneration above referred to does not affect the part of the nerve remaining 

 in connection with the nervous centre, which seems to exert an influence in maintain- 

 ing the nutrition of the nerve. The ganglia, as well as the brain and spinal cord, 

 have been shown by Dr. Waller to be centres of this influence. He found that, in the 

 central and undegenerated portion of a divided spinal nerve, while the fibres belong- 

 ing to the anterior root owe their integrity to their connection with the spinal cord, 

 those of the posterior root are similarly dependent on the ganglion ; and that, if the 

 posterior root be cut between the ganglion and the spinal cord, not only will the fibres 

 which belong to it in the trunk of the nerve beyond the ganglion remain unchanged, 

 but also those above the ganglion, in the portion of the root left in connection with 

 it ; whereas the segment of the same root which remains connected with the cord but 

 severed from the ganglion degenerates. Section of the sympathetic nerve in the neck 

 is followed by degeneration of the cephalic segment as high as the superior cervical 

 ganglion, but no farther. 



BLOOD-VESSELS. 



The blood, from which the solid textures immediately derive material for 

 their nourishment, is conveyed through the body by branched tubes named 

 blood-vessels. It is driven along these channels by the action of the heart, 

 which is a hollow muscular organ placed in the centre of the sanguiferous 

 system. One set of vessels, named arteries, conduct the blood out from the 

 heart and distribute it to the different regions of the body, whilst other 

 vessels named veins bring it back to the heart again. From the extreme 

 branches of the arteries the blood gets into the commencing branches of the 

 veins or revehent vessels, by passing through a set of very fine tubes which 

 connect the two, and which, though not abruptly or very definitely marked 

 off from either, are generally spoken of as an intermediate set of vessels, and 

 by reason of their sinallness are called the capillary (i.e., hair- like) vessels, or, 

 simply, the capillaries. 



The conical hollow muscular heart is divided internally into four cavities, 

 two placed at its base, and named auricles, and two occupying the body and 

 apex, named ventricles. The auricles are destined to receive the returning 

 blood from the great veins, which accordingly open into them, and to pass it 

 on into the ventricles ; whilst it is the office of the latter to propel the blood 

 through, the body. The ventricles have therefore much thicker and stronger 

 sides than the auricles, and the great arterial trunks lead off from them. 

 Each auricle opens into the ventricle of the same side, but the right auricle 

 and ventricle are entirely shut off from those of the left side by an imper- 

 vious partition placed lengthwise in the heart. 



The blood is sent out by the left ventricle into the main artery of the 

 body, named the aorta, and passes through the numerous subordinate 



