124 



HISTOLOGY. 



formed the peripheral nerves. The dorsal root fibers joined it on entering 

 the cord and the motor fibers arose from it; between the two was a diffuse 

 net. In opposition to this conception, the neurone theory set forth that 

 the nervous system is composed of distinct cells, the neurones, which are 

 related to one another 'by contact and not by continuity.' Some even 

 supposed that the nerve fibers were retractile and by breaking their contact 

 produced unconsciousness. In recent years when the syncytial nature 

 of many tissues has been shown and fibrils have been found passing from 

 cell to cell in smooth muscle (?), neuroglia, and some epithelia, it has been 

 reasserted that there is fibrillar continuity between nerve cells. The idea 

 that the nervous system is an intercellular network with formative or 

 nutritive cells appended to it, perhaps comparable with the elastic network 

 in connective tissue, is now rejected. Peripheral fibers are not found to 

 develop by the anastomosis of chains of cells. It is probable but not certain 

 that the connection between nerve cells is merely by the contact of pericel- 

 lular nets and of spiral terminal fibers wound about the cell bodies. 



VASCULAR TISSUE. 



The vascular tissues include the blood vessels and the lymphatic 

 vessels, together with the blood and the lymph. 



BLOOD VESSELS. 



DEVELOPMENT. In an early stage the blood vessels of the embryo 

 form a network in the splanchnopleure. In mammals, as in the chick 



(Fig. 20, p. 21), the .portion of the 

 net nearest the median line forms, on 

 either side of the body, a longitudi- 

 nal vessel, the dorsal aorta. The 

 part of the net folded under the 

 pharynx constitutes successively the 

 vitelline veins, the heart, and the -ven- 

 tral aortae continuous in front of the 

 pharynx with the dorsal aortae. The 

 heart first appears as two dilated 

 vessels, one on either side, which are 

 parts of the general network. They 

 are brought together in the median 

 line under the pharynx and fuse. 



At first the heart pulsates irregularly, but with the establishment of the 

 circulation, its beats^become rhythmical. The blood flows from the net 



FIG. 149. 



Blood vessels from a rabbit embryo of 13 days, 

 developing as endothelial sprouts (en) from 

 pre-existing vessels (b.v.) ; b.c., blood corpus- 

 cle within a vessel. 



