quired to show the formative layers. In the later stages. dissection is required, and is best performed with embryo preserved in spirit. If living embryos are placed in spirit, a natural injection of the vessels may be obtained.

III. ORGANS OF THE BODY.

Anatomists usually group the organs into systems, as the osseous, muscular, nervous, vascular systems, etc., but for histological study a classification based on physiological considerations may be more convenient for the student.

I. VEGETATIVE ORGANS.

1. Nutritive, or organs pertaining to the absorption aud distribution of pabulum, including the digestive and circulatory organs.

The mucous membrane of the intestinal canal contains many follicles and glands, whose secretions serve important offices in the preparation of the food. These will be referred to in the next section. The epithelium of the intestinal canal is columnar, except in the œsophagus, where it is laminated. Beneath the glandular layer of the stomach is a stratum of fibrous connective tissue and muscle fibres in two layers, an internal with transverse, and an external with longitudinal fibres. The tissue of the small intestine beneath the epithelium is reticular connective, entangling lymphoid cells. The structure of the large intestine is similar to that of the stomach. The villi of the small intestine begins at the pylorus, flat and low at first, but becoming conical, and finally finger-like in shape. The epithelium of the villi are columnar, with a thickened and perforated edge (Plate XXII, Fig. 161). Between the epithelial cells of the villi, peculiar "gobletcells" are often found, which Frey supposes to be decaying cells. The reticular connective tissue of each villus is traversed by a vascular network, a lymphatic canal or lacteal, and delicate longitudinal muscular fibres. If the

villus is unusually broad, there may be more than one lacteal. The lacteals absorb the fluid known as chyle. They are blind ducts, and nitrate of silver injections show them to have the same structure as other lymphatics.

The lymphatic radicles are widely disseminated through all the tissues and organs of the body. They take up nutritive fluids, either from the alimentary canal, or such as have transuded from the capillaries into the interstices of the body, mingled with the products of decomposition, and convey them into the general circulation. Hyrtl's method of demonstrating these radicles is by passing a fine canula into the tissue containing lymphatics and forcing the injection by gentle pressure. They are either networks, analogous to capillaries, or blind passages which unite in reticulations. The structure of the vessels has already been described, page 201. Lymphatics and capillaries do not communicate directly. A lymph-canal may be surrounded by capillaries, or run alongside of a capillary, or a lymphatic sheath may envelop a blood vessel. This latter plan is seen in the nervous centres, and has been called by His the perivascular canal system.

The larger lymphatic trunks are interrupted by nodular and very vascular organs, the lymphatic glands. These consist of the reticular connective tissue already described, surrounded by an envelope of ordinary fibrous tissue. One or more afferent lymphatic vessels penetrate the capsule, or envelope, and similar efferent vessels make their exit from the other side. Frey describes these glands as consisting of a cortical portion, follicles, and a medullary portion composed of the tubes and reticular prolongations of the follicles (Plate XXII, Fig. 162). There is a complicate system of communication between the follicles. The afferent vessel opens into the investing spaces of the follicle. These lead into the lymph-passages of the medullary portion, from the confluence of which the radicles of the efferent vessels are formed. The lingual follicular

glands and tonsils, the solitary and agminated glands of the intestine (Peyer's patches), the thymus, and the spleen have a similar structure, and are called lymphoid organs.

In the thoracic duct the epithelium is inclosed in several layers of fibrous membrane. The latter contains transverse muscular fibres. The heart, although an involuntary muscular organ, has striated muscular fibres. These fibres are not, like other striped muscles, collected into bundles, but are reticular. The heart, like other organs, is supplied with lymphatics and blood vessels. The cardiac plexus of nerves consists of branches from the vagus and sympathetic. Numerous microscopic nervous ganglia also occur, especially near the transverse groove and septum of the ventricles. It is thought that these are the chief centres of energy, so that the heart pulsates after its removal from the body. It has also been shown recently that the sym pathetic and vagus filaments are in antagonism, so that stimulation of the vagus interrupts the motor influence of the sympathetic, and may bring the heart to a standstill in a condition of diastole.

The structure of blood vessels has been described under the head of vascular tissue. No special boundary exists between capillaries and the arteries and veins. The arrangement of the capillaries, however, is various, and often so characteristic that a practiced eye can generally recognize an organ or tissue from its injected capillaries. (Plate XXII, Figs. 163 to 168.) For methods of injecting, see page 64. Capillaries form either longitudinal or rounded meshes. The muscular network, etc., is extended, while fat-cells, the alveoli of the lungs, lobules of liver, capillary loops of papillæ in skin and mucous membranes, outlets of follicles, etc., present a more or less circular interlacement. The capillary tube lies external to the elementary structure, and never penetrates its interior.

2. Secretive Organs.-True secretions serve important offices in the organism: as the materials of reproduction;